If you are purchasing it from a LFS why not ask them to hold it for you ? If you're buying it from a local reefer, well, then nevermind :-)
I don't mean to discourage you (or maybe i do), but 3 months is a long time to ask someone to hold a fish for you. What if that fish for some reason dies in their care? That someone can easily say they did everything in their power to look after your fish for you but who knows. Maybe they didn't, or maybe they did and your fish died anyway, you just don't know..
If it were me I would just wait out the three months until my new tank was set up and ready for the Angel. It is a fairly easy fish to ask a LFS to get in for you.
Just my unsolicited opinion
