Deore shifters for my new bike

I just bought a Mongoose Tech-4 (I know) and really like the bike with
the exception of the shifter setup. It has the S-RAM grip shifters and
are really annoying. The system is much better and smoother than the
setup on my Nishiki, but are too easy to accidentally change when
off-roading.

I want to upgrade to the Deore Rapid-fire setup, but not sure what to
get. It seems most everything out there is for 9x3 setups. I have 8x3
gearing with a front disc and rear V-brake.

What is the difference between a brake lever for V-brake and one with
disc? It is a cable-actuated caliper, so I don't understand what the
difference is other than maybe the leverage.

I have found a few setups on Ebay, but they seem to be for all V-brake
or all disc setups. Could someone point me in the right direction or to
a specific model?

Thanks,
Dave

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