I believe you are:
the letter 'a'
See me once, I'm the first of my kind
First letter in the alphabet
See me twice, unhealthy lives we left behind
AA stands for alcoholics anonymous which helps recovering addicts
See me once, because there's only one
use the word a when there is one of an item( a chair, a table , etc.)
See me twice, then there's something we shun
not sure for this part but maybe AA like a scream or shout? People tend to ignore/not help AA and in a sense shun it. Hopefully third times the charm: People in AA shun alcohol?
See me once, some mistake me for a mountaintop
A capital A looks like a cartoonish mountain
See me thrice, without me many devices flop
As pointed out by @El-Guest AAA batteries are commonly used in devices to power them