The mixture is a cup of 1-1/5 = 4/5, which contains equal amounts of orange juice, grapefruit juice and pineapple juice, so in this part, each juice is a cup of (4/5)*( 1/3) = 4/ 15; Orange juice is a cup of 4/15+1/5 = 7/15, so:

Orange juice: 7/ 15

Grapefruit juice: 4/ 15

Pineapple juice: 4/ 15