Step 1:

Start with [State] on rows, and sum([Sales]) on Text.

Step 2:

Create a rank calculation. Rank(Sum([Sales]))

Step 3:

Add the calculation to the filter shelf. Keep only the top 3 values.

Step 4:

The grand total still displays the unfiltered amount.

Step 5:

Drag [State] onto the filter shelf.

Step 6:

Set the filter to be Top 3 by sum([Sales])

Step 7:

Now, the grand total is filtered to only the top 3