Sounds strange, not exactly sure what you mean there. Is there no way you can just tighten the nuts further until it's tight? Even if it squeezes the dropouts in a bit it won't matter, it will just spring back. Or did I miss something?
Putting the spacers on the outside of the dropouts sounds like a bad idea to me - you want the track nut tight on the frame.
If there's a 3mm gap inside, those spacers should fit just right if you put them on the inside of the dropouts, and you won't have to squeeze the frame in every time you mount the wheel.
Sounds strange, not exactly sure what you mean there. Is there no way you can just tighten the nuts further until it's tight? Even if it squeezes the dropouts in a bit it won't matter, it will just spring back. Or did I miss something?
Putting the spacers on the outside of the dropouts sounds like a bad idea to me - you want the track nut tight on the frame.
If there's a 3mm gap inside, those spacers should fit just right if you put them on the inside of the dropouts, and you won't have to squeeze the frame in every time you mount the wheel.