In my scene the model only has three bones attached to it. Most of the vertices have weights for these three bones, but a few of them only have weights for one or two of the bones. For some reasons, I'd like to let all of them have weights for "at least" three bones, and "no more than" three bones, because for now a few of the vertices only have weights from one or two bones.
Bones deform vertices that belong to a vertex group of the same name as the bone. You can manually assign and adjust the weight a vertex has for each group.
While in edit mode the vertex groups panel looks like this -
The assign button will assign the selected vertices to the selected group with the current weight setting. The remove button will remove the selected vertices from the selected group.
In the properties sidebar there is a Vertex Weights panel that can help, it can be elusive as it only shows if a single selected vertex is assigned to one or more vertex groups or multiple selected vertices that belong to the same group (only the one common group will show).