Are you using a ported or sealed enclosure for these? The larger the box the lower max wattage should be so I'm going to figure out for 300w rms a sub so 900 rms total. This will show you haw to wire the subs: http://www.the12volt.com/caraudio/woofer_configurations.asp?Q=3&I=22#results
I'd go the 1.33ohm route and use a single high quality amp like this: http://www.crutchfield.com/p_575P1000X1/Rockford-Fosgate-Punch-P1000XID.html?tp=35834
Now you don't need that much power for them to hit hard but if you're planning on 3 12s then my guess is you want a high SP. Make sure you research the proper box dimensions as that makes a huge difference.