There are several ways to do this:
The Consolidate feature will allow you to build a set of rules that
can Sum, Average, etc. (Note that checkbox that says Create Links if
your original data may change or you will to run consolidate again)
You could write a formula like =Sheet1!A5+Sheet2!A5+Sheet3!A5 (maybe
in A5 of Sheet4) and then copy that to other cells