I used the SB-27 and SB-20 when I had an FM3a. They are both compact and give you TTL, although they are maybe a little less powerful than other speedlights. The SB-27 has a swivel head, the SB-20 doesn't but works nicely with a cord that retains TTL like the SC-17. I vaguely remember the SB-27 was made with the FM3a specifically in mind. I could be wrong. But any TTL speedlight will do e.g. SB-24, SB-25 etc. They are just bigger/more powerful and balance less well on the small FM3a.