Yes salt will help with nitrite posioning, and the level needed is small (something like 0.05% will do) so is not likely to be a problem even in salt sensitive fish & mollies should be able to handle that and more.
However, a better way is simply to eliminate the NO2
. As long as your change water is dechlorinated, temperature, pH, KH
matched to your tank water, you need not limit yourself to 50% PWC
per day. In emergencies, I have done 100% water changes (ie remove the fish to fresh clean water & completely drain & refill the tank). This is a bit stressful to the fish - but no more so than you removing the fish from the lfs
& putting it in your own tank (actually less as your water will be identical except for the lack of toxins).
With the NO3
up in the 3's even after your first water change, I would do a 75% change right away to bring the level to less than 1. Also to be sure to remove as much mulm & decaying food as possible or you'll get another spike a few hours after your pwc
. Once you get the NO2
down to managable level, keep checking & doing pwc
as needed till levels becomes zero (will take a few days depending on how much food is left in tank).
You can use prime at 5x dose, but the priotective effect is only temporary (about 24 hours according to company person when I emailed them a couple years ago). Therefore you still need to do pwc
's to remove the NO2