Let n = the number of nickles you have
Let d = the number of dimes you have
Let q = the number of quarters you have
You have 183 coins so n+d + q = 183
You have 3 times as many nickels as dimes so n = 3d
The total in coins is $15.75 = 1575 cents so 5n + 10d + 25q = 1575
You have three equations with three unknowns. Now solve this.