Having converted three fridges the only way you'll know for sure is to bring the kegs with you. Also don't forget that you need about 1"-3" of space beyond the top of the kegs to accommodate for the bulge in the serving and gas lines.
You may need to take the plastic moulded door panel off the door and replace it with a piece of plastic. You'll gain several inches of space and it's easier to install faucet shafts this way.
You'll probably have to keep the crispers at the bottom of the fridge. The compressor is usually located at the bottom-back of the fridge and this creates a bulge where the crispers are. It's not enough space to fit three kegs, so plan on cutting a piece of plywood and placing it on top of the crispers for support. Keep this in mind when you are measuring because you could lose 1/4" or more of space due to the plywood platform.
If you don't want to lug kegs around with you, take some cardboard and make a box the size of the keg. Add the space you need for tubing and it should give you a pretty good visual. Measuring could work, but you'll never know for sure until you close the door with something in the fridge.