Let the initial investment of A is 3x and B, is 5x, then C investment is also 5x, but most important to note in this question is the time duration of investment
Like, A invested for 12 months, B invested for 12 months and C invested for 6 months.
A:B:C=(3x×12):(5x×12):(5x×6)
=36:60:30
6:10:5