Multiple Interdependent Cells

Hello I have a question about making cells interdependent in Excel.

I have 5 cells that should add up to 100% and 3 that can be manipulated by the user basically:

A
User Input 1
User Input 2
User Input 3
10%
39%

In column B I have a whole bunch of formulas which are dependent on the percentages.

I want to make it so that the user is forced to input something in User Input 1 and User Input 2. While, User Input 3 is dependent on User Input 2.

So if the user puts in 20% for User Input 3, it takes the 20% from user INput 3 (effectively reducing user input 3). i want to make it so that if the total does not equal 100% there is an error, and if one input is negative that it will also display an error.

input.xlsx

this is kind of the idea