Hi

behind the the sofa on ear level is the best position.
5m distance is imho to far for a surround experiance

I agree with @DomB. Near the sofa is by far the best - if you place them 5m away you could well find yourself strangely detached from the sound. It won’t wrap around you over there.

Also, not right behind the sofa. Behind left and behind right (at the outside corners of the sofa or at the walls), at ear level. You should still be able to see them when you turn your head. 

 Don’t aim the 100s directly at your head, in fact, it might be best to face them slightly to the outside shooting toward the outside walls.  The 300s will work better as surrounds.  One reason for that is, as surrounds, the front facing speaker in the 300 is disabled reducing that unrealistic direct rear speaker sound. That creates a fuller more realistic rear sound field.

 I have 300s as rears.  When I listen to Dolby Atmos orchestral music I can’t tell that I have rear speakers at all but it sounds like I’m in orchestra hall.  This is the way I want to hear it.  There should never be any sounds that make it obvious it’s coming from one of the rear speakers.

 My room is a little asymmetric narrowing in the rear on the right side.  So the right rear speaker is closer to me on the rear plane.  Trueplay took care of that.  The sound is well balanced at this point.

Thank you all for your responses.With a little work, I may have a third possibility (option C).There would be about 3.5 m in the center of the sofa and they would be facing each other (see image).Would this option be better?

Yes, that’s where I was referring to when I mentioned ‘at the walls’. That looks like a good spread to create a wide soundstage. 

But don’t make them face each other - they should point slightly forwards, towards those on the sofa.