10W per sq/ft minimum for electric unit heater in an insulated garage. 3 car garage, probably looking at 8,000-10,000 watts, 50A or 60A breaker.
But first, you need to determine how you want to use your heater. Is this a unit you simply want to turn on when you’re working in the garage? Or do you want to set the thermostat to maintain a minimum temperature all day every day?
How quick of recovery time do you want when it’s -10 outside and the wife left the garage door open for the past 3 hours? (pro-tip…interlock the garage door operation with the thermostat. Easily done with a door switch or a relay contact off of the opener.)