You mentioned he wanted something different. Clearly, it wasn't to buckle down raising a family. Would seem awkward staying with his dad. Do you have family to turn to, or someplace to live? I'd recommend getting to family probate court and get your child support arranged.
Being a single mom to three won't be the easiest, but it's do-able.
Do you have access to a counselor ? It's a grieving process. Sorry for your sorrows.