Use the LARGE function with SUM to return the sum of the three highest numbers in a range:
=SUM(LARGE(A1:A20,{1,2,3}))
Replace A1:A20 with your actual range. This returns the largest, second-largest, and third-largest values and adds them together.
If you want a formula that is easier to extend to a different number of highest values, you can use:
=SUM(LARGE(A1:A20,SEQUENCE(3)))
Here, 3 specifies that you want the top three values.
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hth
Marcin