I’m thinking of doing maths, physics and economics for a level, do I also need further maths to do economics at oxbridge?

That would depend on the college you apply for. Some colleges require you to do further maths a-level and some don't. That said, taking further maths would certainly be helpful once you are doing the course since you would have to go through this material (at a faster pace) at university anyways. However, I wouldn't recommend taking a fourth A-level if it means sacrificing the quality of the grades you get in the other three. If you think on replacing one of your choices, only do this if you think you will do as good or better in further maths. Another option is to take further maths a-level as a fourth subject and drop it before taking the exam or once the work load becomes too big.