[SPOILER] Stuck on Cistern stage; valves won't budge

[Serena]

I feel like I only hit the bridge itself with a rock, not specifically the chain. Everyone is so sure it's the chain I'm doubting my memory.

both work if i recall

Another option is to lower the bridge to it's halfway 'stuck' point, and then hit the lever to raise the other bridge - run onto it as its raising and take a running leap onto the 'stuck' bridge - this will lower the bridge as well.

As for the three valves inside the cistern proper, there definitely are three valves.