Size it from the air conditioner’s nameplate, not from the size of the house. Running watts can look moderate; starting watts can be several times higher. r/Generator threads regularly describe a compressor that wants more starting watts than a mid-size portable can give, which is why owners talk about soft starts and about moving from a 30-amp inlet to a 50-amp inlet. This site will not invent a “typical 3-ton” starting-watt number.
Honda’s home table does list one useful nearby figure: a 10,000 BTU window air conditioner at 1,500 W running and 2,200 W starting. That is not a central system and should not be used as a substitute. For central air, read RLA (or FLA) and LRA on the outdoor nameplate, convert to watts at the rated voltage, and enter them as a custom load in the calculator, along with the fridge, furnace fan, and lights you still want on.
See also the soft-start question and the 30- vs 50-amp inlet question. Electric heat or a heat pump in heating mode is a separate, often larger, problem.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.