I like that you are thinking about the footfalls. it's a bit early to really be able to feel them, but to have that in mind is admirable. to your question, it is #3 footfall to which you want to sync your motion.
that is, the footfall wherein the horse lands in the most 'downhill' position, that is #3. in that step he is landing on his leading leg, and it will sound the heaviest. kind of like 1, 2, 3 and . . .(moment of suspension) 1, 2, 3 and . . .
if you get used to listening and feeling for that last , heavier footfall, that is your most downhill one, and it's to that one that you want to try and sync up. think of reall pushing DOWN into the saddle/ground with that down hill landing. think of really having even your feet go straight down into the ground on that leading footfall.
think of it as "1, 2, Down! . . . and 1, 2, Down . . . if you get yourself goind down with the horse, and you ALLOW the horse to carry you up, you'll by rythmically matched. I guarantee it!