I say keep the Arsenal, but start piecing together an AR. Surely you can come up with the scratch to get a stripped lower receiver... you can get one for $75 all day long. Then as you have extra dough, start buying other pieces for it. That way you can build it the way you want right from the get-go, and you get to learn a lot about the workings of your rifle when you put it all together yourself. I would probably just go with a complete upper though, since it's your first build, instead of putting it together yourself on a stripped upper. They are a lot more involved to assemble than the lower. But by all means put the lower together yourself. You can save some money doing it this way, too.
As usual there is a lot of misinformation about the two rifles in this thread. For one, the reason people don't use AKs in 3-gun has less to do with accuracy and more to do with the fact that the AR is quicker to reload, has less recoil, is more optics-friendly, and has better irons. Accuracy factors in, but for the usual target sizes and ranges involved, AK's are plenty effective. My cheap-ass WASR keeps it under man-sized out to 400 yards. AK's are more accurate than most here give them credit. The guy who said they can only hit man-sized targets out to 100m has obviously never shot one... either that or he is a very poor shot, in which case he wouldn't do any better with an AR.
And FlyinBryan, I would have to see this "independent testing" that "happens all the time" which shows that AKs are less reliable than ARs to believe it. And I might not even believe it then, since it contradicts everything I have seen from over a decade of experience with both types of rifles. But if you have a link or some other reference, by all means post it up.
BTW I'm only "coming to the defense" of the AK here because people posted inaccurate comments. No doubt the AR is the better choice for 3-gun, and probably for most other uses as well... but that doesn't mean you will see me getting rid of my AK any time soon either!