Maybe this have been up before but i searched and did not find any and i do not remeber that I have read it before.

For an example, if the x-wing have a target lock on a TIE fighter since an earlier round. And this round the x-wing got a focus action.

First the x-wing throw 3 blank. He use target lock to throw 3 eyes and then use the focus to make that 3 hits.

Is this legal or illegal? If not where does it says it is illegal?

It's legal. You're only limited to re-rolling any dice once, but if you want to change that second result with a Focus token, that's not a re-roll. This is about the best attack roll you can manage on any given turn.

Perfectly legal. One thing to keep in mind: if your original roll had been focus, blank, blank (instead of blank x3) you would want to use the target lock to reroll the blanks first. Using focus only changes current focus results to hits, not any subsequent rolls.