Is it a male then? If it is, I would suggest you don't add any more gouramies - regardless of size. How big is your tank? Three-spots idealy need at least 10 gallons per fish and shouldn't realy be kept in groups of fewer than 3 (singly they are fine - but keeping them as pairs usualy results in the more dominant fish bullying the other to death). Let me just point out that when I say 10 gallons per fish, I'm not saying you can keep a three-spot in a 10 gallon
Three-spot gouramies (that is blue, gold, opaline, platinum, cosby and lavender gouramies) are particularly aggressive. Males are worse than females because of their high level of territoriality.
In a large group or with younger fish, you'd see fewer problems because aggression only realy starts at sexual maturity and, if the fish are in a large group, then the aggression is devided amongst all the individuals - meaning that no single fish is targeted.
Unfortunately, an adult, fully grown three-spot is not a fish that'll appreciate company and mixing him/her will only stress all the fish in your tank unecessarily. Even getting a fish that's the same size will not solve the problem - the current gourami views this tank as 'his/her' territory - newcomers are intruders and are seen as a threat to chase off. Because of the restrictions of a tank, 'chasing' can lead to the death of the 'intruder'.
If you realy want to add more gouramies, stick with the same species (color doesn't matter though - so it could be a gold or platinum for example). There are a few others (generaly larger species) that work with three-spots but it depends on your tank's size (for example kissers, moonlights and snakeskins can sometimes work but require decently large tanks).
If your current fish is male, don't add any more males because they
will fight to the death. You can try adding females but only if you add at least two (so aggression is devided between them) and make sure they are of a similar size. Re-arange the tank before you put them in to make the tank more 'neutral' territory for all the gouramies involved. If your fish is a female, then, again, a couple more females would be an option. Alternatively, you could try adding a male and female.
Keep in mind that, if you have both sexes present, if your fish breed the male will become particularly defensive of his bubblenest and may well kill his tankmates - that includes non-gourami inhabitants.
Like I said initialy, I don't think adding another gourami is a good idea at all and it would eb best to just look at non-gourami alternatives - it's simply not worth the risk IMO.