Falling star target twice to a unit with deflect. How many deflect affected to me?

Yes — you pay the Deflect cost twice if both "Deal 3 to a unit" instances choose the same Deflect unit. Falling Star makes two separate choices, and Deflect charges "as an additional cost for each time they choose me" (Rule 809.1.c), so pointing both instances at that unit means paying its Deflect Value twice.

So a Deflect 1 unit costs an extra 2 Power total (any domain), a Deflect 2 unit 4 Power extra, on top of Falling Star's 2 Energy + 2 Power. If you can't pay a given Deflect charge, you can't make that choice — you'd have to send the second instance somewhere else.