If a "Pro" camera is defined as having the highest possible durability, reliability and being able to take the most advanced accessories, then no, it wasn't.
The FM3a probably has a shutter life of "only" about 100,000 actuations compared to about 250,000 for the "F" models. This can be important if you shoot several rolls of film a day, day in and day out.
Its absolute reliability was probably lower than the "F" models. Having to re-stage a wedding to recover lost shots probably wouldn't help a photographer's career.
It wouldn't take "state of the art" accessories such as 5-6 FPS motors and 250 exposure backs, which, *if* you need them, then you really need them.
So not "pro", but still a very, very nice camera.
P.S. My favorite cameras and the ones I consider best also aren't "pro" cameras by the above definition.