Well, if your middle blue ram slot isn't working and the black slots only work if all of the blue slots are populated, then you will have to use 2 x 8 GB in dual channel mode, or use the 2 x 8 GB in the blue slots and the other 8 GB in the black slot so at least you have 24 GB. I would think 16 GB of RAM would be enough for most applications.
Wait! You say the black slots only work if all of the blue slots are populated, but you also said that you put 2 ram sticks in the 2 blue working slots and one ram stick in the black slot and the pc sees all of the ram. So that would imply the black slots are recognized even if all of the blue slots aren't populated. Have you tried just the 3 black slots to see if triple channel ram is working?
Also, try installing and removing the ram from the "non-working" middle blue slot several times. Perhaps there is some oxidation or corrosion on the ram slot due to it not being used. You might spray some contact cleaner in the slot with the power off to see if that helps.
Are you sure there no bent cpu pins on the motherboard? If there are, that could affect the integrated memory controller on the cpu, causing a ram slot to not be recognized.