Refrigeration is the only real trade left.
The other two have been bastardized into terms for hvac workers, in general, IMO.
Guys that call themselves steamfitters are usually members of the steamfitters onion, and DO work on steam, but probably not exclusively. Sheet metal workers can either be in the same boat as steamfitters, calling themselves that due to their affiliation with organized crime, or they might actually knock and bend tin for a living.
I do refrigeration.