In the cell where the formula should live:

1. Type =average(
2. Click on the first tab that should be included in the formula
3. Click on cell B3
4. Hold down shift and click on the last tab that should be included in the formula
5. Let go of shift
6. Press Enter

The formula will resemble:
=AVERAGE('Tax wk 1:Tax wk 52'!B3)

FYI, Your life will get easier if you will resist the urge to divide data up onto multiple sheets unnecessarily. This is called horizontal partitioning and it leads to trouble.