1st of all 12/8 is not an odd meter, its even, so therefor it should be associated with 4/4.
Each of the 4 beats would be broken up into a triplet. So a measure of 12/8 would be counted 123 223 323 423.
The differnce between 9/8 and 3/4?
You know how when you play a measure of 3/4 you emphasize the downbeat of 1 (waltz feel), in 9/8 each of the downbeats would be emphasized, and again the triplet feel would be introduced over the beat.
This is hard to explain without being able to play it for you, since we are talking about sudtle differences. For an example of 12/8 check out the classic motown song "When A Man Loves A Woman" and listen for the implied triplet feel.
Ciao
