[Lester B. Flat]

C# is three frets down from E, or one and a half steps. There are two half step intervals in a major scale, between 3 and 4 and between 7and 8. Sing the major scale down from E(8 or 1) and C#(6th) will be your third note. E - Eflat(1/2 step, 1 fret) - C#(full step, 2 frets). Total 1 1/2 steps or 3 frets.

To look at it another way, if you are in the key of C#, E is the minor 3rd but still 1 1/2 steps up or 3 frets.:food-smiley-015:

 

[Lester B. Flat]

You could just capo the guitar at the third fret and tune to E with a tuner. Then just remove the capo and you'd have a C# dropped tuning (close enough for rock&roll!). I'd use very heavy strings though with that much of a drop!