How big is your tank?
The tetras are known as fin nippers but not generally against other fish, most their own, if you can get more get them, as long as there is enough tank space?
Between 6 - 10 is the decent sort of size for them, this means that they can establish a pecking order which they will not have when there is only three of them.
Make sure that your tanks has a decent few plants so that the tetras and gouramis for that matter have a place to de-stress and hide/chill out?
Try to get atleast another 5 tetras and if the problem persists decided which species you prefer and take the other back?
Reesy