Whats the difference from 3" and 3' in a measurement?

Question

For example what is the difference from

3" x 12" and 3' x 12'

Thank you for the qualification

Answer 1

3 feet = 36 inches

36" and 36'

Answer 2

So 3" means 3 ft? and 3' means 3 inches?

Answer 3

It's the other way.

" = inches
' = feet