I have done it different ways over the years.
When all 3 were unmarried, I spent the same amount on each child.
When the oldest was the only married and no children I divided his budget between him and his wife.
When younger ds got married and had a baby, I divided his budget between the three. (The baby actually got more of course but that was my starting budget).
Now they are all married. Yds has two children and his gf has one. Older ds has two children. So now, I spend the same amount on everyone. I tell them what I am spending and they decide if they would like a couple gift or individual gifts. If they want a family gift, the children still get a separate gift to open.
Now that, again, is my starting budget. If one of the three needs something, I will get it. Any “extra” gifts for my child or any of the grandchildren will be given to open at their house. At my house, everyone gets the same number of gifts in their age group (so each of my kids/kids in law get the same number and each grandchild gets the same number).
The only person I give money to is ods’s oldest. He is a teenager and likes cash. Besides between two sets of parents and 4 sets of grandparents, there isn’t much to buy him.