On a sub panel there are 3 30A breakers for dryer, water heater, and heat/air unit

Considering that all of them, most likely, will run at the same time
Do I need to run a 100A wire to it form the main box, about 50 feet away or can a smaller gauge wire would be OK?

If you want to run anything else from this sub-panel you definitely should go with a 90 or 100A feeder.

Why are these three majors on a sub panel?

If you want to run anything else from this sub-panel you definitely should go with a 90 or 100A feeder.
I agree!

Why are these three majors on a sub panel?

No idea why but, that's how it was when I got into this house,
I haven't really though about running them back to the main box but I thought it would be cheaper to just run that wire but I guess I have to price it to really know that

And this is a 100A box for 6 breakers, so with 3 240v breakers there is no room for anything else

Maybe it'll be better just to run everything back to the main panel since its not that far away and everything is open

If I did run the 100A feed to it I would need 3 #2 wires correct? Since there is only 3 connections in the box and its still inside the house?