You need to tell us what kind of servos you're using.
For coupling all three servos together, you might be best off using two brackets: one that holds the servos together, and a second that couples their output shafts/splines and the crankshaft. The latter bracket would be something like this
(but you'd probably need to build your own), but with an extension of some sort for the third servo and a further connection/extension to the crankshaft.
If you're using hobby servos, you're probably going to run into issues with the servos not quite having the exact same calibration (aka they'll go to slightly different angles given the same PWM pulse width.)