I found out how to activate the salt dispenser when in 3 in 1 mode.
Their are 2 electronic water hardness settings.
One for non 3 in 1 mode and the other for 3 in 1 mode.
To set water hardness in 3 in 1 mode, switch on 3 in 1 mode, go to the electronic water hardness menu and set it to what ever you want, this will only effect the 3 in 1 mode.
To set water hardness in non 3 in 1 mode, switch off 3 in 1 mode, go to the electronic water hardness menu and set it to what ever you want, this will only effect the non 3 in 1 mode.
AEG customer services and breakdown departments new nothing about the above, I found out by trial and error, the john lewis JLDWW1203 user guide which is an AEG gave me the idea = it sort of said the above.
As mentioned before the AEG user guide simply said the water softner is switched off when in 3 in 1 mode, it gave no mention of how to switch it on.
The john lewis user guide was printed nearly a year after the AEG, I bet thats why it mentions how to do it.
I expect this ability could be on many or all electrolux / AEG / john lewis dishwashers.