hi my baby is 14 weeks this month, also 3.5mths and his intake is also like your baby's - 80-100ml every 2hours.. sometimes slightly more, sometimes slightly less but on average is 80-100ml.
And the even strangest thing is - his intake has been like that since forever... Ok , since he was 3 weeks old till now also same, never increase the amount.
But he still put on weight fine, he is 7kg now. So I dont really bother.
If your baby is putting on weight healthily then you dont have to worry