An average is just the total attendance numbers / the number of performances.
Let’s let the number of people in the third show be equal to x so we can write this as a formula, it’s easier to read. x is just "some number we don’t know"
The average is
Average=Total attendance/3
So
3000=(2580+2920+x)/3
So let’s multiply by that 3 on both sides, make x easier to find
Average*3=2580+2920+x ( * means multiplied)
The average is 3000
So
2580+2920+x=3000*3
2580+2920+x=9000
Then we subtract those other attendances to get x on its own.
x=9000-2580-2920
x=3500