I think Ace3 lib have the /in command, so if you want to yell in 5 seconds, you just write this to your macro: /in 5 /yell for the alliance!
Something like this would work (if the /in command still exists and works)
/petattack
/in 1 /cast yourmountname (for mounting after 1 sec, the waterbolts should be in air by now)
/in 3 /cast yourmountname (for dismounting, the waterbolts should had hit the target already so it's cool to dismount without aggroing)