Seems to be
1251
60 300 1500
simplifies to 1, 5, 25
5*25 = 125, plus the extra 1
Also could look at it from the perspective of 5^0, 5^1, 5^2, or 605^0, 605^1, 60*5^2
If the sequence continued (assuming 1251 is out of the sequence) it would be 1251, 60, 300, 1500, 37500, 187500