Miscarriages... can someone help me

So I'm 19 and had 3 miscarriage.. I have had blood work and scans done the doctor said I might have pcos but its doesn't really look like it and they said my uterus is fine but just to make sure I got a gynecologist appointments February (so long away) but if I dont have pcos then what is the problem I dont really understand why i cant keep a baby if everything is fine with me.. can someone please explain the possible reasons I keep having them ( I am over weight for my age and hight ) but the doctor said it shouldn't be that.. so I dont know. I just want to understand..