"I was told I could just dig them up and plant them in large pots if I want to keep them for next year. Is this true? "

Yup, that would work. One of the easiest ways if you only have a few. Just don't water them until you want them to grow.

"I also heard that Dahlias need to be trimmed in order to grow properly. Mine seem to be growing fine - outwards and upwards, and have 3 or 4 blooms on them each. Should I still trim them?"

Nope, don't trim now! They were probably referring to 'stopping' the plant, which is done by pinching out the middle sprout after 3-5 leaf sets are formed. This encourages the lateral branches to form at each 'armpit' of the leaf sets. No worries, you are good to go!