The only reason I don’t include the center is because of the available spacing, l-r. Also, getting into the acoustic performance of many 2-way centers, you run into potential intelligibility issues and a lot of complications.
*shrugs
Adding a third bookshelf as the center could work, but the lateral spacing is still tight…
The available spacing is no good for two speakers, let alone three!
Now we need to set this as a challenge, especially to you would be builders.
So, this is a corner locations, with all the acoustic effects that go with that location, which are highly significant.
We have to trust the OP that, this is his only location.
What he wants mainly is good dialog, but I'm sure he does not want the background music and other effects a mess.
So we are really confined to a good mono solution. There is a solution and an elegant one, that would solve his problem well.
Now let us see who can come up with the correct engineering solution. You won't find the answer at Best Buy or on line.
I will give you one hint. For inspiration, go back to the dawn of high fidelity audio. There you might find the hint of the correct engineering solution that takes advantage of the corner location to solve the OP's problem.
Shady and Matt please join in.