Id set it up and 'let 'im at it!' Honestly at 3 he's likely to be very capable at getting in and out, and would have to really move around to fall out over the rails already there. Having said that I'd definitely get him to practice and explain it would be dangerous and painful to play up there being silly in case he fell iykwim?? I understand where you're coming from though. We ended up having to put ds1 in a top bunk at around 2 1/2 and I freaked at first too but he was fine. Having said all that you could try get a higher bedrail thing to squeeze between the one already there and the mattress...