First of all, I very much doubt that the actual capacity of the battery has changed that much in a short period of time. I assume that you know the capacity was 3 hours by actually running the computer on the battery for that lengthof time. I this how you determined that the capcity then became 15 minutes or did you believe what the computer told you?
There is no way of determining the capacity other than running the computer until it says "OK the battery voltage has fallen below a certain point and I switching off". Now, along the way the computer will estimate how long is left but this is an estimate based on the history of charge and discharge. Some battery packs now have chips in them that make that estimate. If you are relying on what the computer tells you it was either lying before or lying now.
Some manufacturers web sites will tell you how to recallibrate the battery. This is usually done by running the machine, without any battery saving things like switching off drives, enabled until it determines that the battery is drained. You then recharge the battery with the machine switched off. You then repeat this cycle and the battery is recallibrated. Do check the manufacturers instructions before doing this. HP, I think, provides a program run from a CD that does it all for you. It is probably worth doing this every two months or so.
