I am 3 weeks post op today, when do the steri strips come off? Do I take them off?

It depends. Some doctors use stitches and steri strips on the outer layer. Others don't??

You should just call your doctors office and ask them.

At my post op appointment, the nurse took all but the big one off. But, I have internal stitches so that may be why?

I agree with Jaffe -- call your doctor and ask. Mine were also taken off at the first post-op appointment, even off the one incision that I was told was "extra" large (had three steri-strips!) Since then I have been applying Triple-Anti-B cream to the incisiions and they are healing very well.