I had the same issue with my Gouramis in my 30-gallon tank. The problem is just like ejara said...the Gouramis are very territorial. Any time you have just a couple, they tend to fight a lot. I"ve heard of several solutions.
1-Get more gouramis, and they seem to fight less. I think it's more of a situation where the aggression gets "spread around" rather than focused between two or three fish. The only way you can do this is to get a much larger tank.
2- Remove some of the Gouramis. In your case, even removing the Opal Gourami, the Sunset and the Dwarf may still have issues. (The fish in the background of your picture is a Sunset Dwarf Gourami). It may just be that the Dwarf has spent so much time running away from Opal, that Sunset hasn't had time to get into the mix.
I"ve been able to keep Blue Gouramis and Gold Gouramis in my community tank with no problems, but strictly one at a time. Any time another is added, it's Friday Fight Night. For some reason though, I've never been able to keep a Dwarf Gourami alive more than acouple months, even by themself. Any other type of Gourami grows to be 4-5 inches...Dwarfs are lucky to make it 3 months in my hands. I've given up on them...but to get back to your question...in a 10-gallon, I'd do one one Dwarf only.