Quick question about 12 bar blues progression
 

I seem to be mixed up on some music theory as it relates to 12 bar blues progression.

The chord progression uses a combination of I, IV, and V, right?

I am confused because I see this written out with I called "tonic," IV called "subdominant" and V called "dominant" in lessons on this forum, and on Wikipedia. However, my understanding was that the IV chord is the mediate, and the III chord is the subdominant? (I=tonic; II=supertonic, III=subdominant, IV=mediate, V=dominant, VI=submediate, VII=subtonic/leading)

What am I missing?

Thanks,

~Ken

IV is subdominant, iii is the mediant

Yeah it goes tonic, supertonic, mediant, subdominant, dominant, submediant, leading tone.