Any and all soundbars are a combination of front left, centre and front right speakers. Sonos is no different, so you can’t “add” the speakers he’s asking for. If he wants independent front three speakers you can’t provide a solution using Sonos devices. 
 

For an “ultimate” setup though, a second set of Era 300’s or a pair of Fives set up as a stereo pair will probably give much better stereo music play than the Ultra can do. 

As ​@nik9669a said you cannot add left/right speakers to a Sonos home theater. The only upgrades you can make would be to swap the Arc for an Arc Ultra and add a second sub 4. 

Regarding ​@nik9669a suggest about adding another set of Era 300’s or Five’s as a stereo pair. To be clear they should only be used for music. They would not be part of the home theater setup. *

Even though you could group them with the Arc Ultra for TV audio you’d create a 75ms delay causing an echo effect having them in the same room. On the other hand you can group them to the Arc Ultra for music without introducing any delay.

You mentioned adding wall electrical outlets at speaker height? I assume you were referring to the Era 300’s you intend to use as surrounds. Unless you intend to wall mount them I don’t see the need to place electrical outlets at speaker height. 

One more thought and bare in mind that I have no idea of the actual room layout…

I assume the TV will be placed at one end of the 30 foot length against the wall and the seating area would ideally be somewhere around 8 -10  feet from the screen. Mounting the Era 300’s 22-20 feet behind the listener/viewer on the rear wall is a pretty far distance. Remember, I prefaced the previous statement by saying  I have no idea of the actual room layout.

In any event good look with your project!

 * I have a set of Fives’s in my media room that house my Arc Ultra, Sub 4 x 2 and Era 300’s as surrounds. However, the Fives’s are only used for music as I’m a two-channel stereo purist concerning music.

I’m not sure what “ultimate TV theater and music listening room” means to you or your client. In my opinion a soundbar, from any manufacturer, is not the ultimate, however, soundbars are very convenient, using very few wires — and they can sound pretty good, just not “ultimate”.

Thanks everyone. The room is a rectangle 16x30. AjTrek1 you are correct the couch (viewer) will be about 12’ from the TV screen. I have the option of mounting the rear speakers at about 16’ from the TV on the side walls (4” behind viewer). 

Theatre: 1 x Arc Ultra, 2x Sub 4 and 2x Era 300 as rears.

Music: 2x Fives mounted left and right of the TV at head height while sitting.

I anticipate his next question is can we add two more Fives at the back of the room. Would that really be necessary in a room this size?

Don’t do it. There will be places in the room where you’ll only hear the “left” channel from front-and-rear speakers and other places where you hear the right channel. The result is a total loss of any stereo sweet “spot”. 
A Sub (not mini) bonded to the Fives or Eras will add to the sound for music play though.