I've got all of them with the exception of the Keltec. Well and the PTR, but I think a CETME is close enough.
I'd avoid the Keltec strictly due to the rarity. You can't even find the things, and good luck finding replacement parts if it breaks 10 years from now.
AR-10 probably has the widest variety of available accessories. Used to be that they were rare, but now AR's are all over the place. You can really customize it up however you want. Optics mounting is the easiest of the three if you get a flat top.
If you had asked this question 10-15 years ago, the FAL would have won hands down due to magazine and parts availability. That's not the case anymore. They are still around, but there aren't piles of surplus parts all over the place, and DSA is the only major manufacturer of new ones. That being said, DSA makes very nice rifle and they offer all sorts of accessories and customization options. Plus, I think they look the coolest out of the three. Optics mounting is easy, but you will need to buy a railed dust cover.
M1A's are good if you shoot better with a more traditional stock. The downside is that they are a little more expensive, and optics mounting is probably the hardest of the three.
You can find magazines for any of them for less than $20 if you hunt around. This is one area that the PTR (or CETME) really has an advantage in since G3 magazines are still super cheap.
You are going to spend about the same amount of money for the basic rifle on all three of them, it's the accessories that are going to determine price difference.