Answer with Explanation:

Part filled by (A + B + C) in 3 minutes
= 3 (1/30 + 1/20 + 1/10)
​= (3 x 11/60) = 11/20 . 

Part filled by C in 3 minutes = 3 /10. 

Therefore,
Required ratio = (3/10 x 20/11) = 6/11