Hey, can someone help me understand the 12 bar blues? I've seen a bunch of videos and read a bunch of stuff, but I'm having trouble with it. Like, I thought you use three chords, but it seems like everyone I see doing it is using more than that? Idk...sorry, I know this is a dumb question, but please help.
Yeah, you use three main chords; the I, the IV, and the V. Everybody is adding stuff to it, though. Think of it like a musical template- it would be boring if everybody just played the same shit, wouldn't it?
Calymos is right. 1,4,5 is a very common progression.
Here are a few examples.

E - E, A, B

G- G, C, D

A- A, D, E

You don't have to play just the chord, you can do simple things within the scale to spice up the progression.
I'm not really sure of what you're having trouble with--could you list some specific things you want to know?

At its most basic level, 12-bar blues uses only three chords--usually the I-IV-V of a given key (let's use C to keep it simple). Using the key of C, those chords would be C (the tonic), F (the subdominant), and G (the dominant). Each "bar" is actually a quarter note in 4/4 time, with each measure consisting of four chords; each chord corresponds to a particular bar. Symbolically, the 12-bar blues looks like this:

1 2 3 4 5 6 7 8 9 10 11 12
I I I I IV IV I I V IV I V

What that means is that you're playing the corresponding chord with its requisite measure. Rewritten with the chords instead of their key positions:

1 2 3 4 5 6 7 8 9 10 11 12
C C C C F F C C G F C G

So basically, you're playing a C chord four times, then an F twice, a C twice, a G once, then an F, a C, and finally a G before repeating with the C. You can make adjustments like substituting 7th chords (C7, F7, and G7 in our case) for a bluesier sound or you can just move to another key entirely.

I don't know if I helped at all but here's a Wiki article about it that might give you more answers. Good luck!

http://en.wikipedia.org/wiki/Twelve-bar_blues