Hello there. To do it cheaply is easy! You can easily find a good approximate value by performing your discipline (running cycling etc) for 12 minutes.

Therefore get on a treadmill and, once warmed up, run as far as you can for 12 minutes.

This test is called the Cooper 12 minute test, and you can find tables on the web - just google 'cooper test' and this should bring up what you're after.

Swimming is a different matter, as the laboratory way of determining your VO2 max is by using "Douglas" bags and a tube which analyses your geseous exchange. I haven't a clue how to do it for swimming so i would get an average of the readings for the other two disciplines.

Hope this helps. chris