ANYONE PLAYED A 12 STRING BASS?

How are they tuned? Are the three strings the same as a power chord? So the “E” string is accompanied by a “B” and an octave up “E”?

Correct I believe. The idea is it produces a chorus like effect, so it sounds like you have three basses instead of one. A situation where this might be useful is in three piece band (guitar, bass, drums). The guitar goes into a solo leaving the bass and drums to hold down the rhythm, causing the sound to thin out (unless the guitarist has a loop pedal, which I always thought was cheating a little bit). Four piece bands avoid this problem by having a dedicated rhythm guitar but in a three piece, the bassist could play one of these and add to the fullness of the sound. Or at least that is how it was explained to me.

A lot of the time the two octave strings on the 12 will be tuned off pitch a bit - one just a bit high, one just a bit low. As The Bassist said, it gives a fuller sound.