German rams are best kept at 28-30c which isn't compatible with your other choices.
Dwarf gouramis are very territorial, males moreso than females. But you might struggle to keep the peace with 3 in a 3fter. However with plenty of plant cover it's certainly worth a go, you just need to be aware of it.
The only bettas (siamese fighting fish) you can keep together are females. And they would need to be in a slightly larger group than 3, 4-5 would be better. It allows them to build up a pecking order.
Thing is gouramis and bettas come from the same family of fish, anabantoids, they occupy the same level of the tank as eachother and are very territorial between eachother. Unlike with risking the gouramis, I would not risk mixing bettas with gouramis as they are practically always a nightmare. However again it would be up to you, so long as you're aware of the possible problems and will be ready to seperate them all if neccesary then you should be fine.
As for the black knife fish, sadly no. They grow far too big for your tank.