Let's say you have labels @Computer , @Phone , @Car , and perhaps many others, and you want find tasks that have labels @Computer and @Phone , but not @Car  or any of your other labels.

First, open your Smart Lists from the top-right ☰ menu:

Then click to create a new Smart List:

Give it a title, then scroll down and click to add an Advanced filter:

The way to create this filter is to tell Marvin that you want @Phone and @Computer and that you want exactly two labels:

You should see tasks having these labels and no others in the Preview.  If so, then click to save.  You can now filter with this Smart List, work on it from the Matrix or in the Master List, add it as a tab in your Plan Day pocket, etc.

Should you want 3 labels, use @A @B && @C && numLabels 3 == &&` .  If you're having trouble creating an advanced Smart List filter, feel free to contact us and we'll get you sorted!