I just googled the stepper motor model number, and it threw up a few different datasheets/catalogues.

The catalogue for MAE stepper motors is http://www.probyte.fi/pdf/HY200-3424-0170.pdf
Which confirms the 3A/75V ratings.
However another datasheet put that exact same model at only 2.1A per phase.

I can't find any definitive figures for maximum temperature, but up to 80degC seems to be a common recommendation. If you are running the motors below their ratings, I wouldn't worry too much about it.
If you're not happy with them running that hot, try what Lucan suggests, and drop the current setting further.