I don't know about Russia, but, "In the United States, most colds occur during the fall and winter. Beginning in late August or early September, the incidence of colds increases slowly for a few weeks and remains high until March or April, when it declines." - kidsource.com

You basic assumption about winter is in doubt.

No. 1 has some credibility...  I do believe that low humidity does help to kill germs in the air, on surfaces, and in towels and kitchen sponges when they dry out completely. *

No. 2 makes some sense, since if a cold runs through all the people you are in contact with, they can't catch the same cold again.

No 3. There's no evidence that being cold gives you a cold.

* A petri dish has a lid after all, and if the dish dried out completely, that would ruin most germ culture experiments. Last week, with 15 degrees F outside, the humidity in this house went down to 12% on a digital hygrometer.

http://www.kidsource.com/health/the.common.cold.html

This is a question that many people have ponderd. Not all the answers are consistent but I will have a try at answering your question and make some (it is my wish) relevant remarks.

The cold is more a set of symptoms than a disease. Most colds, like influenza, are caused by infections with viruses. The common cold is a collection of symptoms which are usually caused by rhinovirusses or coronavirusses.

These infections are usually caused by inhalation of the virusparticles and are usually self limiting (they go away by themselves).

It is believed that when the weather turns bad, people tend to go indoors (to where the fire is, for example) and huddle together. If somebody sneezes, a great amount of virus particles are dispersed and inhaled by those in close proximity (the name influenza is thought to be derived from people influencing others).

The remark about the immunesystem is probably correct, people with a suppressed immunesystem are much more vulnerable than "healthy" people.

Sunlight contains ultra violet radiation, which has an anti-bacterial effect. Dry air which is rich in sunlight is "cleaned" by nature because the conditions for micro-organisms to thrive are not optimal (no moisture, lots of UV).

The best way to prevent getting a cold seems to be staying away from people who have a cold, going out into the sunshine whenever possible and a healthy life (food and exercise).

Finally it should be pointed out that the cold amounts to much labour time lost, it seems unlikely that a vaccine will be developed, partially  because there are so many different viruses and there does not seem to be a common serotype. And there is much discussion wether viruses are "alive" or not.

Debates on cold and colds are hardy perennials, and the subject will not go away any time soon. For one thing there is far too venerable and voluminous a body of received folk wisdom to confuse the issue. Secondly the situation itself is intrinsically highly complex and multifactorial, involving hundreds or thousands of species of virus, some groups of which are effectively totally unrelated. Furthermore, our reactions to the viruses are extremely complex, varying according to genetics, ambient conditions, and even personal histories. Perhaps the most obvious effect is previous exposure and immunity; isolated populations such as Eskimos actually have been wiped out on occasion by epidemics of colds innocently brought in by symptom-free outsiders.

However there are other non-obvious effects as well. People speak of "shaking off a cold", when they do nothing of the kind. Cold viruses typically establish their nucleic acid within our cells, and if we survive an attack, we simply suppress their growth and the symptoms of the disease they cause till further notice. When our resistance drops at various times in the future, such as in times of stress, we may well develop "a cold", but it is not a new cold, just an old subversive element re-emerging. One does of course get new cold infections from time to time, but they are the minority.

There is merit to some of the points you make. For example, desiccation does kill some percentage of some kinds of microbes, including many viruses, although one cannot rely on such an effect for projection. The effects of cold on the body are inconsistent, but some of them are highly relevant to the spread of respiratory infections. For example, catarrh, coughing and sneezing favour the spread of mucus, not only through the air, but more importantly, on hands and on objects we handle. Some people apparently manage to avoid winter colds and flu to a remarkable degree simply by systematically avoiding hand contact with their faces. It makes sense in principle at least.

An interesting and important point is that the most important cold viruses only grow at a reasonable rate in relatively cold tissue. The mucous membrane lining our respiratory tracts is of course constantly cooled by evaporation, and the relevant viruses tend to do better at 25°C than 35°C. Accordingly the bodily defence of raising the temperature to about 40°C makes a lot of sense. Do not do anything to lower your fever until it goes over 40°C, and in any case do not force it down to your normal bodily temperature unless they happens to be a special reason. This is why the best ways of treating colds and flus all involve warmth, rest, isolation, and sleep, lots of sleep. Aspirin and similar means of lowering fever are to be avoided until fever becomes actually threatening.

In the light of such considerations it is hardly surprising that some of the best means of avoiding infection involve covering the mouth and nose with something to filter the air and keep it warm. It also reduces the amount of mucus that you disseminate, and the amount of mucus from others, that you expose your own mouth and nose to. Purists point out that viruses can pass through far smaller passages than those through your scarf or filter, but that is irrelevant. What is far more important is the fact that the viruses abound in mucus, and do very little drifting about as isolated particles in the air. Silly ideas of that type are why purists are so heavily prone to colds. A scarf over your mouth and nose while you rob a bank, can protect you from recognition and reduce your chances of infecting the counter staff with your sniffles.

Exactly why particular ranges of low temperatures apparently inhibit the spread of colds, I could not say; it is a cold day in South Africa when the mercury drops below -10°C! Certainly, once a virus is properly frozen, freezing it down to liquid helium temperatures is unlikely to do it much more harm. Whether reduction of the temperature of our mucous membrane to below a comfortable temperature, and reducing the amount of mucus secreted inhibits the development and spread of the viruses, I do not know, but it does not seem implausible.

bearing some of these principles in mind, and closely watching the behaviour of your body and your colleagues, you might find yourself able to make better sense of colds, and their epidemiology.

good luck!

Because colds are caused by bacteria in the atmosphere. As we all well know, bacteria don't tend to produce well in the cold. Less bacteria means less chance of catching a cold.