MyDMX 3 works as a fixture selection based program, so select all the fixtures you wish to control, and then the general tab is where you will get access to common channels like dimmer, and if they color mix RGB picker, for all the other functions you'll have to go to each tab and adjust.
Watch my guide to programming on 3.0 coming from 2.1:
https://youtu.be/Fz8V8nOA3I8