Do I have to fulfill the Questbridge regular decision form completely?

I just got accepted to the college that was my first choice, but it was after I turned in the Questbridge regular decision form with all of the other schools that I intended to be backup schools. Since this happened, I don’t want to continue completing each of their applications since there were quite a few of them. However, there were about two or three of them that I still don’t mind applying to. Can I just not complete the other school’s requirements?

Yes, but you should send an email to all the schools on your Regular Decision Form that you intend to withdraw your application.