There's several issues going on here.
1. The water softener can/ could cause problems. Some say it doesn't, some say it does. But ultimately, it removes Hardness and adds salt. I believe you can bypass the softener by using an outside spigot.
2. GH is a measure of magnesium and calcium in your water. I believe Wonder Shell only adds calcium but you'll have to do some research on that. I'm pretty sure though and I think it will raises KH also.
3. Although a 3dGH is on the lower side, it's not too low. However, snails and shrimp would be a bit more comfortable around 4 to 5dGH. 5dGH is where I'd go and that is fine for tetra as well.
4. Your KH is really high. I would look to drop that to 3dKH.
Grab a sample of pre softener water and test GH and KH just to see where your numbers are.