It wouldn’t disqualify you to take further maths as one of only three but it would be ideal to have three as well as it as many unis would see the former as a bit too narrow since it’s not really three different subjects.
If you don’t like physics you could always take something like politics, which is more related to economics than physics and would give you a bit of a break from maths-heavy subjects.
Basically, maths, further maths and economics is a fine selection but if you want to go to particularly good unis then it would help to take another subject as well, such as politics or business, to show you can deal with the breadth of three different subjects.