Ypur son needs time with you without having to so closely share you with his sister. Others have given you good ideas on how to arrange this.
I have some ideas on sharing. It rarely works between siblings during the preschool years. As you said they are at different developmental stages. You can provide your daughter with toys appropriate for her age. When she grabs for her brother's toy, trade with her for one of hers. You can teach your son to do that too.
I've always had difficulty doing too much talking. I've finally learned to not give explanations some of the time with my grandchildren. I always did talk to much with my daughter. Talking dilutes your request. It sounds like your request is open to discussion or that you're not sure. Especially if you talk and don't follow thru with a consequence for the child when they don't do what you say. In your situation you'll find it much easier if you let go of the rule that everything has to be shared. Then there's nothing to explain after you've explained the new way of doing things a couple of times. Trade with the baby when she grabs your son's toy. Keep them separated as much as possible for the short time you play with your son. It's true at 9 months that she wants and even needs to get into everything. That is how she learns. But she also needs boundaries.
Perhaps you're having difficulty similar to one we have between my granddaughter who is now nearly 8 and her brother who is now nearly 4. My daughter handed down her toys to her brother. But she still feels that they are her toys. They also share a bedroom. My granddaughter sometimes cries, saying she has to share everything with her brother and she has nothing that is her's exclusively. In part this is because her mother insists that they share everything most of the time. I don't think it's a good idea to always insist on sharing. They do need to learn to share and my two grandchildren are at an age that it should be easier but it's not. Children need to feel that they have their own things and that it's OK to have boundaries about what has to be shared and what doesn't. This includes some alone time with Mom.
At 3 your son is not developmentally ready to easily share. He's just now learning that he has ownership and the ability to control his toys. One has to feel secure in the knowledge that what's their's is their's and that they will get it back before they'll be able to share it. They also have to be able to be patient. Three is not patient. My grandaughter at nearly 8 is not patient much of the time. Babies and young children do not have a good sense of time. If you think about it, time is long at age 3. One month is 1/3 of his life.
Nine months is usually developmentally able to spend some time entertaining themselves if they've been given the opportunity to learn how to do that for themselves. Both of my grandchildren loved the saucer. It's true that you still need to be aware of what they're doing but you dont need to always be involved with them. Another thing that my granddaughter liked to do was to sit in her high chair and pick up cheerios to either throw on the floor, in her mouth, or sometimes into the dish. Toys that the baby can manipulate will keep them busy for awhile. Stacking cups, stacking rings, blocks or sitting on the floor surrounded by simple things like pots and pans, plastic dishes a wooden spoon. A couple of small stuffed toys that she can put inside a dish or pan.
Set her up to play so that she can't see her brother. Baby gates are good to keep sibs separated and babies in a safe place. Since you now have a history or fighting over toys it will take time for you to develop a successful routine. You can do it!