Looks like you could take the longest cable and test all three drives from the center plug. Place the cable in the center plug and into each drive.
IF none of the drives work swap cables IF all the drives work then it is a cable problem IF one works and two don't then drives.
THEN if you have bad drives , sic um cowboy , (;-) TP
I will assume that by saying "longest cable" you mean the ribbon cable. The ribbon cables are all the same length. In addition they are not long enough to go from X drive to Z plug in the BOB or Z drive to X plug in the BOB. But from the Y position (center) the ribbons can reach all plugs in BOB
As per picture above, the center plug & ribbon works. So I will remove this ribbon cable and drive and put them aside.
Now I will install the X & Z drives, one at a time, in the center position and test the other 2 ribbons in all 3 plugs in BOB.
Nicolas