If R=red disc and B=blue disc, the possible outcomes are RRB, RBR, BRR. The probability of each can be calculated:
RRB: (5/9)(4/8)(4/7)=10/63
RBR: (5/9)(4/8)(4/7)=10/63
BRR: (4/9)(5/8)(4/7)=10/63
So the probability for each outcome is the same and the total probability is the sum of these: 10/21.