Keep the FM2 & add either an FA or FE2 would be my choice. I would forget the FM3A due the the price premium. You could probably get both an FA & FE2 & have money over for a lens for the price of an FM3A.
With regard to the FA. Program mode is quite primative. Only 2 programs that switch automatically based on the focal length of the lens (AIS lens). Being an FA owner, I wouldn't call it a selling point & hardly ever use it. The shutter priority mode has been much more useful to me. The matrix meter is primative by todays standards but I have found it quite good. Supposedly can be fooled by turing the camera on its side. Have never felt the urge to test for myself.
I have an FM2 as well & wouldn't part with either of them. Find the FM2 & they way that it handles & slows my photography down a joy. The few times that I have used flash with the FM2 it has been with either an SB-15 or 16 & I have been happy with the results.
The FE2 has manual & aperture priority as well as TTL flash. It is another nice camera that would be worth considering.