I’m quite happy playing over the Blues, I know what sounds good and what doesn’t just from experimenting with my improvising other the years but I don’t understand why in a major blues, there are three dominant chords?
Any help appreciated as I’m currently trying to expand on my theory knowledge 
Very interesting question!
Here is very short and incomplete answer that I am borrowing from my jazz teacher of 20 years ago:
Dominant chords come from the hybridization of the minor pentatonic scale (say – C minor), which was used for singing melodies in some African styles of music, with the diatonic chord progression I-IV-V coming from the European tradition (say Cmaj-Fmaj-Gmaj). basically the major 7 would clash with the pentatonic tones for the C and F chords, and had to be replaced by the flat 7.
Interestingly, the major and minor 3rds instead did not clash that badly, and in fact using both (and every micro-tone in between) is one of the key sounds of blues 
That makes a lot of sense, thank you!
In case you’d like to dive a little deeper:
Here are several more articles on the topic by the same author
From a theory perspective, using dominant chords for the I, IV, and V is all wrong, since the V is the only dominant interval, but the blues does many things “wrong” that sound so right to us
Yeah it took me a while to wrap my head around!
Thinking of it as a typical I, IV, V with the flat 7ths as “colour” makes the most sense to me
My reasoning was that if you had a 12 Bar Blues in E7, when the A7 comes around A minor pentatonic sounds wrong because of the note C. In relation to the A7 it would be a minor 3rd which you think would work in a Blues but I would argue that it doesn’t because you are still hearing E7 as your root, so you’re actually hearing C as a minor 6th which sounds really dissonant over an E7 chord
In that case it may be helpful to switch to the major pentatonic or Mixolydian over the IV chord. Switching to Mixolydian over the V chord makes the most sense, since it’s the fifth mode, but different guitarists have differing philosophies over which one to switch to, and when (I myself picked this up from a lesson by Carl Verheyen).
I honestly wouldn’t think of it as something that fits with in a typical western key or in the sense of how the chords in the progression relate to each other with in the context of a key center, and rather would think of each chord themselves as having their own key center or as substitutions. Adapting a chord by chord basis starts to make more sense when you start to embellish the progression further with substitutions that wouldn’t exist in such a neat package as a western key center.
Yep I agree, my go to would be A Major Pentatonic, A Dominant Pentatonic, or A Mixolydian over the IV in an E7 Blues, avoiding the A Minor Pentatonic and A Dorian (though you could still play E Minor Pentatonic and E Dorian)
I am mostly onboard with this and I’d agree that Blues doesn’t fit neatly into western music theory boxes but I think both approaches can be helpful
If you treat it like a I, IV, V with flat 7ths for colour and E7 as the root it’s simple to see why you can solo over the whole progression with E Minor Pentatonic and E Dorian without a problem.
A cool example of playing with this sort-of approach is Clapton’s Crossroads solo, it’s an A7 Blues and he starts with A Major Pentatonic over the A7 and when it goes to the IV (D7) he switches to A Minor Pentatonic. Very cool simple idea and gives the solo it’s own sound!
I think whichever way leads you to a desired outcome, is the right way.
I guess, thinking about this some more, it probably makes the most sense to come at this question backwards.
Think of a bunch of blues riffs and what notes are used in them. I’m typing this on a quick break from work with no guitar handy, but go back to listen to say a bunch of Robert Johnson shuffles, or even Muddy or Howlin Wolf Chicago blues standards, and you’ll find a LOT of riffs built on some variation of root, minor and major third (often in rapid succession), 5th, major 6th, and minor 7th. And, it’s pretty typical to just move that whole pattern up a fourth to cover the IV chord. The V should be dominant anyway in classical harmony, so the other two chords were generally implied with riffs rather than played outright as chords, and often have harmonies that would fall into a m7/m7maj6 for a minor or Dorian flavor, or 7th if the major 3rd was being utilized or implied (via a bent note, slide, whatever).
Like, how many famous blues songs really have some dude just strumming an I7 and then IV7, vs a driving riff that just outlines those chords?
I would say a lot more than you may be giving credit for but I see your point. If you are talking about the root 5, root Maj6 root minor7 da-da-da-da-da-da-da-da riff or similar, then yes it is purposely ambiguous, because you do not imply a 3rd, so anything that contains a P5, M6, and min7 (cough cough Dorian, and Mixo) will work equally well there. And since the 3rd isn’t implied, both the M3 and m3 will work, together and interchangeably because you have nothing that implies either. Also because the half step intervals (2nd and 6th) are avoided in the regular minor pentatonic, that works equally well over that same riff.
There are also examples of strict minor blues that adopts iv and v chords.
You’ve got me second guessing myself, so I’ve got the Robert Johnson collected works on right now while I work. So far it’s a lot of low string riffing with upper register fills, but, well, I’m only four or five tracks in, and this is the complete works with a bunch of alternate recordings, so I’ve got a LONG way to go. 
By the time you get to early Chicago blues though, nary a chord to be seen, thanks to the bass/drums accompaniment and how that opens up arrangements a little. Tons of White Guy Blues/Rock ™ that’s I7-IV7-V7, but imo that doesn’t really count, lol