I follow the rule never hit a multi card hard 16 and im curious if i should be concerned with the same thing as a multi card hard 12 against a 2 or 3.
A 3-card 12 vs dealer 3 would be very much like a 3-card 16 vs dealer 10. A 3- card 12 vs dealer 2 not quite as close a call. (I'm assuming you meant you'd never hit a 3-card or more 16 vs dealer 10. This certainly wouldn't be true vs a dealer up-card of 7. And of course we're talking basic strategy, not when counting.)
Now when its a multi card 12 against 3 or 2 you would do what the index says to (if you were counting) right?
Yes. If you're counting it doesn't matter if you have a 5-card 12 vs dealer 3, you make the correct index play.