I shot in low light with the EX3 using 0DB gain. It looked good in the camera LCD and on the 8.9 Manhattan HD LCD which I was using for focus. When I looked at the clips in the browser they were darker than the shoot. Is there any possible reason?

1-did you calibrate either live moitor?
2-what were your exposure settings on the EX3, auto or manual?
3-how well calibrated is your browser monitor?

The other thing to check is whether the pedestal setting on the live output of the camera was set to 7.5 while it gets recorded at 0. Playback of 0 pedestal material will look darker on the monitors set up properly for 7.5.

Is it possible that your EX-3 viewfinder was set too bright?

It could also be that as you were viewing and monitoring while shooting in a Dark place that when you play the clips back in a bright place or room that the footage appears relatively dark.

I would suspect that it's something to do with the monitor you are using for clip browser. Have you played back from the camera to see how the playback looks on your LCD?