Charges of three long charged cylinders centered on 1 axis

[Omar FTM]

The figure shows a section of three long charged cylinders centered on the same axis. Central cylinder A has a uniform charge qA= +3q0.
What are the charges of qb and qc if E is 0 on point >
a) 1 ( between A & B )
b) 2 ( between B & C )
c) 3 ( out side the cylinders )
My solutions were based on the concept of vectors
a) qB= +3q0, qC= -6q0
b) qB= +3q0, qC= +6q0
c) That's impossible. ( the other choices were impossible )
But they were wrong.
Could you explain what's wrong with my solutions ?

 

[Omar FTM]

I got a) impossible , b)qB= -3q0, qC= +3q0 c) qB= +3q0, qC= -6q0 > They are right. :)