What is the best way to change proportions?

I would like to find the best widget(s) to change proportions.

Let's say I have 2 to 12 parts that sum up to 100%. I would like to create a widget where the user could those parts as they want.

For example, if there are 3 parts, the user can set the first part to 20%, the second and the third part to 40%. If the first part is increased to 30%, the second and/or the third part should be decreased, so that the sum still matches 100%.

Do widgets like that exist and if not, what would be the best approach to deal with it?