Well I don't really think this post is about a routine as much as it is about having someone interfere with your parenting.
What would work best is to have your husband tell his mother to back off.
Routines are preached about and everyone has their opinion on what you should do, how you should do it, but realistically, routines are set for the needs of YOUR FAMILY...
At three months old a routine is good to have, it is healthy and necessary. She may think it is pointless because it will change, but this routine will change as your baby changes, not because someone tells you it is point less. It is not up to anyone to tell you the goods and the bads about the routine you are setting, not even any of us here. The routines can only come from what works best for your individual family....outlaws excluded.
Having your husband to tell his mom to butt herself out and trust she raised her son well enough to raise his own kids...will come much better received from him than from you. He can tell her that if you and he want her opinion you will ask for it, until then all she is required to do is love her grandchild, your son, and keep her opinions to herself.