A.W.
The easiest thing I have found with my children is to skip the toddler bed, and put his mattress directly on the floor. Since he is 3, and seems to need more space, you could go straight to his twin mattress (or whatever size bed he will have) on the floor. Then, add box springs after he has adjusted, then, add a frame. We never used side rails, as my kids were essentially trained on the floor until they stayed on the mattress. Good luck.