So I have three masks. Morokei I carry on my person all the time. I put one of the others in a chest in Breezehome, while I placed the third mask on my alchemy table. Now, that last one is missing. I thought we could put things in different places in our houses for decoration or something.

Is there a way to get it back? I mean, do the dragon priests respawn with their masks so I can jut go get a replacement?

Your horse ate it...

Check Lydia's inventory to see if she has it.
NPCs pick up things like weapons and armor.