The way to solve this is to use the information to create some equations.
The sum of three numbers is 88. -> x + y + z = 88
The second number is 3 times the third. -> y = 3z
The third number is 7 more than the first. -> z = x + 7
What are the numbers? Now you have three equations and three variables and you can solve the system of equations to find the values of x, y, & z.
The easiest way to do this is to substitute out x & y in the first equation.
Replace y with 3z (Because y = 3z), and replace x with z - 7 (Because z = x + 7).
Thus x + y + z = 88 becomes z - 7 + 3z + z = 88, which simplifies to 5z = 95, then to z = 19.
Since y = 3z, and z = 19, y = 3*19 = 57.
Since z = x + 7, and z = 19, 19 = x + 7, and x = 19 - 7 = 12.
You can check your answer by determining whether 12 + 57 + 19 = 88 .... which it does.
