I just want to clarify that the sleep group I went to was not sleep school, it was a two hour meeting with other parents and a nurse. We watched a video (Safe Sleep Space) and discussed tips to help our babies settle and sleep.
I also cut down one of DS's feeds overnight, but only after 6 months. He was waking three times a night which, for me, was too much. For three nights I would cuddle him to sleep for the middle of the night wake up instead of feeding him. On the fourth night he didn't wake for that feed.
3.5 months is too young to do something like this, but if you find you are still having trouble once they are older, it's just an idea that helped me.