3,8,13,?
You should be able to figure out the 4th term now
a + (a + d) + (a + 2d) = 24 => 3a + 3d = 24 => a + d = 8 => a = 8 - d .... (1)
a(a + d)(a + 2d) = 312 .... (2)
Substitute (1) into (2): (8 - d)(8)(8 + d) = 312 => (8 - d)(8 + d) = 39 => 64 - d^2 = 39 => d^2 = 25 => d = 5 or -5.
Case 1: d = 5 => a = 3.
Case 2: d = -5 => a = 13.
It's left for you to find the fourth term of the series in each case.