I do the same, but it's not like a 3-piece won't prevent air from flowing back in. It will suck in the liquid first, but air follows. I use breathable bungs, which still suck in air when you relieve the pressure like Jonathan says, but absent opening it in a space flush with CO2 there's really not much you can do. On the other hand, it is not just O2 but exposure time. When I crack a carboy and rack it to a CO2 flushed keg, I don't worry about O2 carry over. I'm pulling from the bottom, filling from the bottom, and there isn't much mixing. I wouldn't stress about it anyway, just be aware of what you are doing in case you decide to change things up.Do the S-Types really cause oxidation? I'm brewing small batches so long term oxidation isn't really a problem since most batches are gone pretty quick.
The s-locks allow air to flow both ways. If some air gets pulled back in, it could. Personally, I prefer a 3 piece then use a solid stopper when crashing and moving.