Is SO3 always a gas?

Yes

Yes, at room temperature SO3 is a gas. However, it can be liquefied and frozen.

No, SO3 (sulfur trioxide) is not always a gas. At standard temperature and pressure (STP), which is defined as 0 degrees Celsius (32 degrees Fahrenheit) and 1 atmosphere (atm) of pressure, SO3 exists as a colorless gas. However, when cooled below its boiling point of 44.8 degrees Celsius (112.6 degrees Fahrenheit) or compressed, SO3 can condense into a liquid or even solidify into a white crystalline substance.

If you want to determine whether a substance is a gas, liquid, or solid under specific conditions, you can refer to its phase diagram. A phase diagram is a graph that represents the relationship between temperature, pressure, and the physical states of a substance. By examining the phase diagram of a substance, you can identify the conditions at which it exists as a gas, liquid, or solid.