In most cases further maths will not be treated as a separate A2 subject and will not count towards your AAA in the conditional offer.
Just remember that should you drop a subject after you hit submit on UCAS, you have to update each university so they know, as your application will be judged on the merits of the form. So even if you get an offer, if you drop FM before your exams, you still have to let the university know and they will be within their rights to withdraw or amend their offer as they see fit.
And yes, cash in general studies - nobody will care and it just makes life easier if everything is declared just in case things get complicated later down the line.