Try:

/target MyTank
/cast [mod:ctrlalt] Dispel Magic; [mod:ctrl] renew; [mod:alt] flash heal; [mod:shift] Abolish Disease; Greater Heal

Also, make sure nothing is bound to the mod + key in your keybinding settings.
For example, if that macro is on 3, make sure shift+3, alt+3, ctrl+3 aren't used for anything else.