If you're set on Economics as a degree, do Further Maths at AS only, firstly because it's a bit tough at As let alone A2, then a lot of unis only actually accept it to AS level, and I personally think three subjects at A2 is best in terms of workload etc (i.e. the three you've chosen already).
Chemistry would definitely help if you want to keep your options open though, it's generally seen as a good subject to take.
Have you thought about Physics, Business, Computing, IT etc as other possible options?