A Sonos sub will only connect (Bond) to one speaker or two when configured as a stereo pair. You cannot bond a sub to three speakers. Even when a sub is part of a Home Theater; which consists of a soundbar and two speakers used as surrounds, the sub is only bonding with the sound bar.

That’s a shame. 
I have managed to create a group and got it to work 

thanks 

This means the Sub is still connected to the stereo pair, to which you have grouped another speaker?

I think that’s what Iv done yes