Only if the middle is a single enclosure stereo speaker and they are all 8 Ohms.

Center speaker options:

https://www.crutchfield.com/shopsearch/stereo_speaker_dual.html?&pg=2&fa=1#&nvpair=FFCategory||rank11999]Home%20Speakers&nvpair=FFSubcategory||rank101500]In-ceiling%20Speakers

I have three JBL Control 30 speakers, and they are being mounted on an exterior wall. So from what I am reading I cannot do this?

There is no Sonos device that will drive three speakers from another company as left/center/right in a home theater setup, if that is what you’re looking for.  
 

The Sonos Amp, which you appear to have, can drive two speakers, and creates a faux ‘center’ channel using the right and left speakers. 

Thanks for your reply.

This has nothing to do with home theater. They are outdoor mounted on a wall in a large courtyard. Three speakers in a line.

The Amp is incapable of sending both a stereo and a mono signal at once.  So your middle speaker would have to have stereo inputs and mono output, aka a single enclosure stereo speaker. 

Thanks!

Is there anyway to make this work?

One Sonos AMP

Three JBL Control 30 speakers

Is there something I can add inline to the third speaker?

No.  Essentially, if you combine the two channels into one wire pair, you will have one channel powering back into the other, which at best will cause a fault in the Amp, and at worst it can render it a useless, smoking shell.  The only solution is to purchase a dual enclosure stereo speaker as mentioned above. 

You could put two speakers on the right or left channel and one on the other.

You could then try stereo where you’d have a 2/3 - 1/3 channel mix or go to mono on the Amp and probably get even sound by using the channel balance.