The 3a has strap lugs, the 3b does not, as has been said. They have otherwise identical bodies (I'm fairly sure of that) so it should be possible to "transplant" body shells if you really must have lugs.
My experience of the VF is that my 3b has the brightest of all my FEDs but I may just have been lucky with my example. They certainly vary though, I don't think quality control of the VF/RF glass was very tight! And yes, they have all been cleaned so I'm not just comparing dirty with clean.
Of my FEDs, the 4 has the dimmest finder, followed by the 2. That's not counting the 5C, which has a clear VF with a small RF spot so it's not a fair comparison, it's constructed differently. If you want a big, bright, 1:1 finder get a Zorki 4k (one with the shutter speeds not worn off though!). Cheap, capable, lever-wind and it'll do all that a FED 3 will do.