In Microsoft Excel, how shall we calculate in cell A3 (Sheet1) the addition of A3 (Sheet2) and A3 (Sheet3)?

There's a professional more dynamic way to do that.

There's a feature in excel called Consolidate and here's a tutorial on Microsoft website.

let me give you some ways we can do this.

1)

Sheet1!A3=Sheet3!A3+Sheet3!A3

2) if the two sheets next to each other I mean (sheet2 and sheet3)

Sheet1!A3=Sum(Sheet2:Sheet3!A3)

see!!! :)