Answer
The initial charge of each bubble was $~~+4e$
Work Step by Step
Let $q$ be the initial charge of each bubble.
After bubble A hits the ceiling, the charge on bubble A is zero.
After bubble A hits bubble B, both bubbles have the same charge.
The charge on bubble A and bubble B is $\frac{q}{2}$
After bubble B hits bubble C, both bubbles have the same charge.
The charge on bubble B and bubble C is $\frac{\frac{q}{2}+q}{2}$
When bubble C hits the floor, a charge of $-3e$ is transferred up through the wire. Then the charge on bubble C just before hitting the floor must have been $+3e$
We can find $q$:
$\frac{\frac{q}{2}+q}{2} = +3e$
$\frac{q}{2}+q = +6e$
$q+2q = +12e$
$3q = +12e$
$q = +4e$
The initial charge of each bubble was $~~+4e$
