Have you tried using extra lubricant?

Is it the full penetration that hurts her or the girth?

For the penetration there are positions you can do where she is not getting the full size, I have had problems with my dh and he's 7". Some women have shallow uteruses or ones that are tilted. Different positions can really help.

If it's the girth, she needs to relax. A woman can give birth to a 14" head, so there should be no problems with 3"