From the feedback, looks like you don't need to do anything other than turn off the phantom (as you already know).
The V3 has 2 gain settings - low (the -20dB jumpers Tim references) and high. Low might be used for running line-level signals into the V3. High is used for running mic-level signals into the V3.
Tim suggested you should use the low gain, line-level setting if you're running a preamp in front of the V3 (because the external preamp may raise your mic level signal to line level). Some others chimed in saying, yes, you could, but it's not absolutely necessary.
I'd try your setup with the default high gain setting first. If you have problems (doubtful), then you can switch to the low gain setting (and if you do, be careful with the screws on the backplate).