“When attacking or defending, hits scored by air units cannot be assigned to submarines
unless there is a destroyer friendly to the air units in the battle.” (Rulebook, page 30)
Can You Move Chinese at the end of a turn into another chinese territory that already have 3+ guys on it?
Yes. There is no restriction on movement based on the number of units - only on placement of new units.
If there is currently 1 infantry on a chinese territory, can you place 3 more on that territory or can you only place 2?