Just a few thoughts;
First, 5.45 is not too small for 3 gun. It's perfect, whether you're shooting it out of an AK, or an AR. I've run 5.45 AKs in 3 gun several times, and it's a hoot. Magazines will be a little rough right now.
Second, you're not going to find a rifle worth owning for less than an AR-15. It is the most saturated, best supported market in tactical rifles in history. Shop the used market, buy components, put together your own... especially if you buy unfashionable parts (carry handle uppers, for example). Watch the classifieds like a hawk, get on some 'email me when back in stock' lists, and be patient. I think you could easily put together a rifle for $700 or less right now.