Answer:
1/5
Step-by-step explanation:
This are the possible outcome
RRR BBB YYY
RBY RYB RYY RBB
BRY BYR BRR BYY
YRB YBR YRR YBB. Total 15
R red
B blue
Y yellow
Probability = numbers of required outcome/number of possible outcome
No of possible outcome =15
No of required outcome = 3 red marbles
So
3/15
P(3red marbles) =1/5