The season pass consists of 6 DLC packs. If you buy it now, you can have all the DLCs when they come out, and it could be cheaper now. Otherwise you might be able to buy the packs separately and choose which packs to get, but no discount. CMIIW.
Smokeldoggz, I'm right there with you, the wording seemed vague to me as well. The Strangereal Edition infographic, though, does explicitly say three additional missions (plus the three additional aircraft), so I would think we're getting three missions. We don't know if they'll be on maps already in the game or on new maps, though.
I should mention that the Season Pass will be available for purchase after the game releases, and each of the missions and aircraft will also be available for individual purchase. If you want to wait until they start releasing those DLCs, I wouldn't blame you.