Well I suppose the real question here is "can I get in with 3 A-levels if 2 of them are maths and further maths", because I can't see how the content of A-level physics would be useful in any way.
The answer is yes - it is 3 full A-levels, it might be less work than if it were 3 distinct subjects but preparing for the MAT and/or STEP is going to be tough anyway. Also, many applicants won't have a choice in the matter - e.g. if their school teaches further maths as a completely separate subject and they're limited to 3. Hence why admissions staff generally avoid discriminating based on this.