Short of using javascript, I’d create another model just for this purpose. That model have three conditions that with the following logic. 1 OR 2 OR 3
Each condition would be Id “in” field model of another model field Id.

Thanks for that answer. The conditions have changed on my end, though.

Is it possible to get an aggregate count of records from three different objects?

No,  it is not possible to aggregate data from multiple sObjects. 

Digging for another answer and I see this question.  Things have changed!

Make a UI only field that aggregates the length of each model together.  Something like this. 

{{$Model.myModel1.data.length}}+{{$Model.myModel2.data.length}}

That should get you your aggregated value to put in a tab…