New comers in a started battle, surprise??

[Kerilstrasz]

A battle between X and Y is at it's 3 round, when a Z aware of X and Y
enters the battle while X is aware of Z and Y is unaware of Z...
Is Z able to take a surprise round against Y ?
Is Y going to be flat footed against Z for any reason until he becomes aware of him?

I d like for you to quote from books to justify your answer plz...

[Deophaun]

Is Z able to take a surprise round against Y?
If you have the Rules Compendium, this is answered on page 71.

No, because surprise rounds happen before regular rounds begin. Since regular rounds have already occurred, there can be no surprise rounds. However, they do get to go first in the initiative sequence, with their initiative checks being considered 1 higher than the highest result of the original participants.

Is Y going to be flat footed against Z for any reason until he becomes aware of him?
Since Y does not know Z is there, then Z is hidden from Y. Use the exact same rules. If Z is able to continue sneaking up on Y before attacking, then yes, Y is considered flat footed against Z's attacks.