Can I run a electric range, dishwasher, washing machine, and central a/c on 100 amp service? My water heater, dryer and furnace is gas. The proposed a/c unit will be 3 ton. If not, what should I upgrade my service to? Thanks in advance.

Solong as the wiring is correct you should be fine. And most of the things should be on a separate circuit so your OK.

Especially considering the gas appliances you will be fine. I know many people with small homes and 100A services that run central A/C.
A new 3T system will be very efficient as well.