33 weeks pregnant with no.3. Currently on amoxicillin for UTI and had sex with parter and now have a gross fish smell in vagina. NO chance of an std! ?

Vaginal odor. Unprotected sex carries a risk of STD. That being said foul smelling discharge can be caused by a bacterial overgrowth due to a change in PH of the vagina. See a doctor.
See your doctor. You may have bacterial vaginosis (an imbalance in the vaginal bacteria), which has been associated with preterm labor.
Here are some ... During the #3 pregnancy at its 33rd week, it's important to make sure of no STD like chlamydia, gono, trich, etc. So, see gyn-doc timely.