Lets think they have cards, Alex = x , Gil = y and Meg = z. Then,
x+y+z = 740
Alex had 20 fewer cards than Gil. So,
x+20 = y
Alex had 120 fewer cards than Meg. So,
x+120 = z
x+(x+20)+(x+120) = 740
3x + 140 = 740
x= 200
y= 220
z= 320
So Alex had 200 cards, Gil had 220 cards and Meg had 340 cards