One way:

='page 1'!K67 + 'page 2'!J45 + D19

In article ,
Millington wrote:

I have a sum formula in cell K67 for page 1, a sum formula in cell J45 on
page 2, & a sum formula in cell D19 on page 3. What formula would I enter in
cell J19 on page 3 to add the sum formulas from the 3 previous mentioned
pages?