My DS has now had 9 operations.
IME it's best to be informative and matter of fact. There is no real reason for a 3 year old to get scared in advance because they don't have pre-conceptions.
I would ask if he remembers going to the Dr about his testicle (or whatever you said to him) and that the Dr would like the hospital to adjust it a bit and make sure that it works properly / is more comfortable - or whatever. A basic , simple, but factually true explanation.
Say that it's best to do these things when he can't see it or feel it so they make him fall asleep in a special kind of sleep called an anaesthetic so that they can fix him without him being able to feel it. Tell him that the special sleep happens when they give him some special air to breathe through a mask, or maybe they will put some magic cream on the back of his hand which means that nothing can hurt his hand and then pop in a little tube for the anaesthetic to go down.
The main thing is that you speak of it at all times with confident matter of factness whenever he is in your hearing. And IME it is best not to re-assure them. Re-assuring suggests that ther eis something they need to be re-assured about and immediately gets them anxious! Just be calm and factual.
The staff will be really bgood when you get there. the starving can be the gardest part, but they usually do the youngest children first. Break his routine so that he doesn't notice the time he would have had breakfast - for e.g if he normally dressses in nis bedroom and then has breakfast, take him down into the front room fro TV and then dress him there before whisking out the door. And don't eat or drink in front of him in the morning.
Good luck - kids are extremely hardy about surgery and the parents bear all the anxiety.