I ii ii IV V vi vii
E F# G# A B C# D#
The I is E, the IV is A, and the V is B. All you have to to is count from the root chord of the key (Think E=1, count 1 2 3 4, then 1 2 3 4 5 to find the I, IV, and V chords, making sure that they have appropriate sharps or flats depending on key sig.)
So the 12 bar would go
E E E E
A A E E
B A E B
Keeping in mind that sometimes the last bar is E, and sometimes the second bar is A, and there are lots of other possible substitutions, but that's the basis.
