In order to accomplish this, your "all three" indicator's data expression will need to be a bit more complex. It may not need to include every feature, however.
First, you should identify all the fields your other dashboard elements use to filter these layers.
Then, in your expression, consider using GroupBy to group each of the three layers by the fields in question, returning a count for each unique combination. Then just merge your layers together into a single FeatureSet.
The resulting FeatureSet should have any fields available for your other elements to filter, and by presenting the sum of the count field, would effectively show the total of filtered features.
I'd give an actual expression, but it probably depends a lot on your dashboard and the input layers. Feel free to give specifics, and I could try writing something up. Or if the layers are public, could try to work with them directly.
- Josh Carlson
Kendall County GIS