The reason that I don’t know it is the blue unit is completely in front of the red and green one. I know Red has LOS. So for me it’s a question for the experienced people here 
Yes, blue unit can remain in contact with the green unit and declare a countercharge, and the red unit can declare at the same time that it will join the charge and make it a multi-charge.
First figure out which side the red units leader point is in, as that will determine if red unit will end up on the left or right side of the combat. If it is perfectly center, choose a side.
first slide the blue unit to the side it has been determined to be in. since the blue unit is larger than the red unit, blue will lay claim to a larger part of the green units frontage. I would guess it is somewhere about 60-70% so it will slide until it covers 60-70% of the enemy front. then proceed to the red unit. it will try to make contact with the enemy in the space available to it, and fill the remaining space on the enemy front. a unit without fly would struggle to reach this space but since your unit has fly it will have no trouble getting into place.
Your mention of the word disengage only muddies the water - if you meant the optional rule “withdraw” then you do not need to do that in this situation, simply declare a countercharge while engaged like stated earlier. if you did mean disengage as in perform a move back order, then that would prevent the unit from declaring a charge order. in any case since the UK and european scenes play without the optional withdraw rule, I tend to answer as if the rule does not exist, like I did in my top answer.
Thank you for your answer. You assumed well, I didn’t meant a move back. Indeed without fly or nimble, It would be hard to hit the unit on the free spot. My knowledge about the game has risen again
