Typically, when GCSE results unrelated to ALevels are required, it’s a 5 (or even 4, ie., Pass, at lower level universities) in English and/or Maths. There’s no such requirement for English and similar, since an A* in ALevel English is assumed and Maths GCSE is irrelevant to success. This varies depending on “course” and there’s now a new AS Level Maths that can be added to 3 ALevels and and EPQ (not sure how the the AS Level Maths will be used in terms of subject qualifications or evaluation.) Oxford expect 8+ GCSEs with a majority 7+ but they don’t really care if a student got a 6 or even a 5 in an irrelevant subject. In addition, there are contextualized offers meaning the school overall results, the area’s socio economic situation, the student’s individual hardships, etc , are all taken into account. In other words, results from a private (“independent”) school are not reviewed the same as results from a state school where the high aspirational goal is having a 5, many don’t even have 4s, and 6s are uncommon.
None of this would apply to OP anyway, since OP is enrolled in the US system.
For British pupils: yes, if you have 3 unrelated A-Levels, US universities will welcome you. You can even apply “Undecided” if you don’t know what you want to study! They generally DO expect A Level Maths for STEM majors but sometimes not even that depending on theirselectivity and your standardized test scores (SAT/ACT).