24
The series formula is A(n) = n2 - 1
12 - 1 = 0
22 - 1 = 3
32 - 1 = 8
42 - 1= 15
52 - 1 = 24
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63
The next number in the sequence 3-15-6-30-21 is -207.
I'm guessing your sequence is 1, 3, 6, 10, 15, ... In which case it continues: 21, 28, 36, 45, 55, 66, ... (These are the triangular numbers.)
5
The next number in the sequence would be 25.