Yes I'm comfortable using my IIIf without an auxiliary viewfinder, particularly if I am using the Summitar; the in-built viewfinder window is fixed at the 50mm field of view (as you probably know) and it's quite usable. As a 'viewfinder' the built in is nothing like that on my M4 (and no doubt miles away from your M3!).
But, the focussing window is magnified and it is easier to quickly reach accurate focus. When I am in a hurry I just focus on the object of interest and shoot, without delaying to move my eye to the viewfinder window. It works fine.
I hear what you say about compactness. I love the IIIf and just the Summitar for that. And despite owning and using (a lot) the D600, Fuji X100s, FM3a, Leica M4 and Hexar AF (I must be mad - now that I have listed them this has gotta stop!) it is the IIIf I love taking out and running a roll.
As advised above, get a good one.