You can’t. The maximum surrounds you can have is two.

As per the screenshot, the only additional device you can add to the Family Room is a 2nd Sub.

Appreciate your quick reply Sir, 

The app unfortunately showing this option only, i was hoping someone have a idea how to fix it,

 thanks anyway 

There is nothing to fix as it’s not broken.  There is no such thing as a “middle rear surround” audio channel in any of the common audio codecs and thus no reason for Sonos to enable bonding in a 3rd surround speaker.  There would be no audio for it to play.

This is why one frequently hears references to “5.1 surround sound”.  I have not yet heard or seen a reference to “6.1 surround sound”.  The Sonos soundbar deals with the front left, right and centre.

Dolby DTS ES Matrix and Dolby Digital EX Matrix are a 6.1 surround “standard” (note I put that in quotes as whilst there is a specification for it, it is not very common).  They provide an artifical centre rear channel which is particularly useful in diagonal room configurations as it’s difficult to place the two standard rears in an evenly-spaced configuration behind the listener.  I’ve had this setup at home for a number of years with a surround amp that supports this specification, the two standard rears are in opposite corners of the room and the centre rear is in the corner opposite the TV.  The centre rear helps to fill the big gap between the standard rears and works surprisingly well.
(Note that there is also a DTS ES Discrete 6.1 which provides a true center surround but I’ve never seen any audio tracks encoded with this).

I am thinking of possibly moving to an Sonos Arc, sub and surround configuration and would love to have three surrounds but it sounds like this is not possible with Sonos.

Especially since Sonos also only does 5.1 DTS….