The sum of a, a + 3d and a + 5d is 100.
a + a + 3d + a + 5d = 3a + 8d = 100
a + a + d + a + 2d = 3a + 3d
From the data provided we know that 3a + 8d = 100 but this is not sufficient to be able to determine the value of 3a + 3d.
The sum of the given terms cannot be determined with the data available.