Dwarfs are relatively peaceful so you can keep an all-male group or an all-female group without problems as long as you don't overcrowd - and if you go for a mixed group, there should be more females than males (ratio of 2 females to every male is good).
Having said that, opaline gouramies (trichogaster trichopterus) are an entirely different matter. Both males and females will bully other gouramies. Though males are worse, a single female can be almost as bad.
As dwarfs only reach a third of the size of an opaline, it would not be a good idea to keep a single opaline with dwarf gouramies. She'll spend all her time harassing them.
As your tank is about 45 gallons, you could consider getting another couple of female opalines (or any other color morph - blue, gold, platinum, cosby or lavender) and then also adding 2 male dwarfs. The other three-spots will occupy your current one and hopefuly distract each other from the smaller dwarfs.
Having said that, it's risky to add 2 more three-spots as they have to both be female and you must make sure they are the same size as the one you have. If not, one will end up dominating the rest and you'll have other problems.
That's also the other issue here - 3 female three-spots is a good number but, idealy, if you don't want any single fish to be bullied, you need at least 4. The only way for you to do this would be to skip the dwarf idea altogether and get 3 more three-spots (you choose the color morph(s) but make sure they're all female). This last option is what I suggest you do.
To add to my attempt at persuading you to give up the dwarf idea
, you should know that they often carry disease and are becoming increasingly fragile and prone to sudden death. Three-spots (opalines, blues, gold(en)s, platinums, cosbys and lavenders) are, on the other hand, the hardiest gourami and also come in several lovely colors. The only problem is size - be very careful not to over-stock the tank as each three-spot can get to 5-6". You should also allow for about 10 gallons per gourami. Non-gouramies are ok though.
So to some it all up, one opaline female and 2 dwarfs (whether male or female) is a risky combination.
4 three-spot (opaline) females are a better option and skip the dwarfs altogether. You can combine color morphs as long as they are all female and the same sort of size over-all.
3 female three-spots with 2 dwarf males also might work but there's no guarantee so I would do the above instead.
Three-spots get rather large so care should be taken to not over-stock. Don't add more than 5 gouramies of any kind to your 45 gallon tank and avoid male three-spots like the plague (for the moment at least).
It would help if you'd mention what other fish are in the tank and how many of each as you may find some of the above doesn't apply under certain circustances. Ask if you have questions of course.
Ahhh Sylvia methinks we are very much along the same wavelength in regards to gouramies.
If you can't get them returned tonight I'd suggest either using a second tank for them or the opaline or put the opaline in a breeders net overnight. If you have neither option you are going to have to hope for the best really. Assuming all is well return the dwarfs or rehome the opaline. A tank of the size Sylvia mentioned is too small for opalines really. Infact anything but the croaking/sparkling gouramies are too big.
Not on here of course.
