You are sadly one of the few firms that carry any foam concentrate - the increasing majority of service providers replace rather than refill so the type of concentrate is no issue for most firms!
If you can't safely get close enough to a fire to use a 3 litre extinguisher then it may well be beyond safe first aid attack with any equipment. There is a big difference between the test fire in the well ventilated high ceiling lab wearing full PPE and actual fires in buildings. The well known Threshers video at the end shows a fire well within a skillfully applied extinguishers capabilities in an area where the heat and effluent output would be damaging and ultimately fatal to anyone trying to tackle it.
Whilst 6 litre extinguishers give you a bit of a range advantage, many potential users in some work sectors object to the weight.
I've users that have effectively and safely used 3 litre extinguishers on actual fires and they are in my opinion fit for purpose in certain environments.
However if you aren't convinced then it's understandable for you not to stock them - as a responsible provider you are right to only supply items you have full confidence in.