My sister and I are 4 years apart. Growing up it was frustrating for me having to always "watch your sister, look out for your sister, be nice to your sister", etc. It may be helpful to set aside 1 to 1 1/2 hours of dedicated playtime for the sisters.
Get loads of games, dolls, and toys they can interact and play with together. After that time is over then they can go their separate ways. Let them play in a neutral area, not either's room.
When setting play dates for your 3 year old, don't feel obligated to include the siblings of her playmates. A 2 hour play date is great 3 hours if you are inlcuding a meal instead of just a snack.
Set activities you do that your 3 year old can help with. My son would help sweep the kitchen floor and pick up the trash after. I got him a small broom and mop set. He loved them. I would take some onion skins and drop them on the floor by "accident" and he would run and get his broom and sweep it up. He would wash his hands when done and help me cook. He is 15 now and still is helpful in the kitchen.
Let the 3 year old do things for the baby, like bring you the diapers, wipes, bottles, clothes, powder, etc. This should keep her distracted from the 7 year old.
I hope this helps and God bless you all.