I have three data sets and three fitting functions.
I want to fit three curves in the same time, because these fitting curves share the fitting parameters.

The figure is one example.
I have three data sets (blue, red, green).
Blue ones should be fitted by the function y=ax^2.
Red ones should be fitted by the function y=ax+b.
Green ones should be fitted by the function y=bx+2a.

In this kind of situation I want to find the best a and b.
normal "Findfit" allows only one functions.

Does anyone have ideas to solve?