How many MyQ garage door openers can ADC handle

Before I install my 2nd MQ garage door opener,to ADC can I add a 3rd in the future? We have a 3 car garage

There is no real limit.

The MyQ Internet Gateway (828LM) can monitor and control up to 16 MyQ devices, and the MyQ Smart Garage Hub (821LM, 821LMB, 821LMC-S, MyQ-G0301, MyQ-G0401-ES) can monitor and control up to two (universal) garage doors each with a door sensor and up to 16 MyQ devices. There is no prescribed limit to the number of LiftMaster gateways/hubs that can be added to an account.

Clarifying. I am not adding the gateways or hubs. I am installing brand new com plete garage door openers. Can I have 3 on my ADC account?

Yes. Each one has its own Gateway built in.