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The best option will not be cheap. A mini split system, or any portable unit, will not be sufficient to cool an un-insulated, three car garage. Which is at a minimum 10,000 cubic feet. With three sheet metal garage doors that absorb heat, and allow huge amounts of air, to bypass them. A small unit, will not be much better than fans, and cost more in electricity.
You will need to insulate the exterior walls, and as much of the attic as possible, replace the garage doors with insulated doors, or use some type of adhesive insulation on them. Seal the doors, to reduce the air flow. Either increase the size of your existing HVAC unit, or install a separate dedicated 3 ton unit, for your garage. Install duct work, with return air. This will exchange the air much more efficiently, and you can install a thermostat, which will allow you to shut it off when not in use. Again, this won’t be cheap, but I believe you will be disappointed trying the less expensive options. JMHO