Step 1: Total capital of firm.
Capital of A = Rs. 45,000.
Capital of B = Rs. 15,000.
Capital of C = Rs. 20,000.
Total Capital = 45,000 + 15,000 + 20,000 = Rs. 80,000.
Step 2: Calculate capital ratio.
A : B : C = 45,000 : 15,000 : 20,000.
Dividing by 5,000: A : B : C = 9 : 3 : 4.
Step 3: Adjust to new ratio.
Since C is admitted for 1/4th share, his ratio = 1/4 = 4 parts out of 16.
Remaining 12 parts are to be shared by A and B in 2:1 = 8 : 4.
So final ratio = A : B : C = 8 : 4 : 4 = 6 : 3 : 2.
Final Answer: \[ \boxed{6 : 3 : 2} \]