The correct answer is $\boxed{\text{B) 7 : 2 : 3}}$.
Initially, A and B share the profits in the ratio 3 : 1. This means that A gets 3 parts of the profit and B gets 1 part of the profit.
When C is admitted, he is given $\frac{1}{4}$ share of the future profits. This means that C gets $\frac{1}{4}$ of the total profit that A and B would have shared.
C takes $\frac{1}{4}$ share from A and B in the ratio 2 : 1. This means that C takes $\frac{2}{3}$ of $\frac{1}{4}$ share from A and $\frac{1}{3}$ of $\frac{1}{4}$ share from B.
Therefore, the new profit sharing ratio is 3 + $\frac{2}{3}$ : 1 + $\frac{1}{3}$ : $\frac{1}{4}$ = 7 : 2 : 3.