I’m just not sure and want to get full value from feh pass
FEH pass can grant 3 heroes, but I’m not sure if buying it today will work out for you (as you will have only a few hours to get the 3rd hero and do the quests). I would rather wait a couple of days to not risk it.
It is better to get the FEH pass on the 15-20 so you always have 5 days to complete everything.
I believe if you buy on the day of release of a new Resplendent you will only get 2 as the subscription would end on the day before the release (in this case the 9th next month would be your last day). So wait a day (or week) and you should be good.
No, he can have the third hero next month if he buy the FEH pass today, but he will only have a couple of hours to get it and do the FEH quest. So yes, waiting for tomorrow (or a bit later ) is the safest way to go.