Hi Depthseeker
Pretty new to this myself, and agree with Sickbugs on the mental limit thing - when timing my PBs I try not to look at my watch until I think I can't hold any longer - and then add 5 seconds. It can get a bit uncomfy, but the next time you try it, you'll get to your old time OK, and then add a few more seconds.
Look up static tables on the forums, plenty of info out there. I've been doing a co2 table day 1, rest day 2, o2 table day 3, rest day 4, and kept on repeating this for a couple of months. It's a slow process, but over about 2 months I've gone from a PB of 2:45, up to 4:31 - a few seconds at a time. (I know these are not big numbers, but I'm pleased with the results!) By the way, I'm training dry, sitting down, and hungry (which helps a lot I find).
Also doing apnea walks at the gym on a treadmill - constantly walking at 5kmh, starting at a 10 sec hold, breathe for 1:30, then hold for 15 sec, breathe for 1:30, 20sec hold etc, up to (for the moment anyway!) a max 45 sec hold - big lactic acid build up in my lower legs at this point! Best to let someone at the gym know what you're doing too!
All this has made my pool work (with a spotter of course) feel a lot more comfortable.
Hope this is of some help in beating the 3:00 mark.
Ady