Tpinky’s suggestion will work.
To be on the safe side (the classic & recommnded way) will be to define each field properly:
=Sum(Sheet2!A3 + Sheet2!A4 + Sheet3!A1) etc.
What I have done is to click on the SUM button, go to the other worksheet, click on the 1st cell, hold down the control key, select the next cell, then hit enter. I was able to pull in the total from that.
Hope that helps,