The only difference is that the built in model has a fan that blows Ir across the coils. You'll also need to get some pvc pipe to bridge the gap between the kegerator and the counter. I think it's 1.25" schedule 40. Just cut it so that it sticks up slightly higher than the counter. Then caulk it in place in the hole in the top of the kegerator. By the way, cutting that hole in the brand new countertop (or at least getting it lined up) was scary. That's why I had someone else do it.
On the higher/commercial end, there is probably plenty of under counter models that will accommodate 3 kegs. But, as you have probably seen, they're pricey. They're also pretty noisy when the fan kicks on. However, now that I have it set up, I wouldn't want it any other way.
