Bones yellow over time mostly due to calcium loss and drying. Your skulls are, undoubtedly, more fragile than they were some time ago, but not so fragile, I would think, that you couldn't try bleaching. You just have to be careful. The nasal bones and the front of the skull especially are going to be weak spots that you need to watch.

3% peroxide is entirely too weak for what you want to do. You need to try a peroxide cream of something like 30% to really get down into the skulls. Watch the peroxide around your antlers. It will discolor them.

If worse comes to worse, you could always paint them.

Once you're done, I'd suggest you coat the skulls with a thin white glue or clear, flat sealer/adhesive to help keep all those small bones together.