The total ratio representing the number of marbles is 3+5 +7 = <<3+5+7=15>>15
From the ratio, the fraction representing the number of marbles that Brittany has is 3/15, which is equal to 3/15*600 = <<3/15*600=120>>120 marbles.
Alex has 5/15*600 = <<5/15*600=200>>200 marbles.
If Brittany gives half of her marbles to Alex, Alex receives 1/2*120 = 60 marbles.
After receiving 60 marbles from Brittany, Alex has 200+60 = <<200+60=260>>260 marbles.
#### 260