You can have more than 3 variables but in the dropdown list, in one placeholder.
(user choose from predefined variables)
Yes, because each VARIALBE need to be filled in the prompt input, so we don’t want to end up in 10 VARIABLES and your whole screen are input fields (or dropdown lists) you need to fill. 
May be 4 VARIABLES would be available in the future, if that’s the bottleneck, as the UI is not affected with that additional one.