Determine roughly how much weight you need to carry, divide by 3, then go find some castors which will take that weight. eBay is a surprisingly good place to find low-cost quality castors.
For the lifting up/putting down, thing, I'd recommend building a frame for the castors which is normally off the floor, which is lowered and locked in place by a lever which puts the castors down for moving it... hard to explain, I'll see if I can find a picture later.