Gourami are labyrinth fish, just like Bettas, and IMHO should be considered on similar lines. It is ok to keep one alone, but equally be wary of putting two males in the same tank, unless it is a large tank with plenty of hiding places.
I have two honey gourami in a 120litre, one is very much larger than the other, and presumably the boss, but I have seen no animosity between them. The smaller one is quite retiring, spending most of the day swimming through the plants, but happily comes out to feed (once the initial frenzy has died down). The larger one is bold enough to feed from my hand. I see the two of them pair up regularly, one following the other around...but as soon as the "dominant" one swims away, the shy one will retreat again.
3 spot Gouramis can potentially grow to about 15cm, I don't know the size of yours now, but as you intend, a larger tank will be necessary for them to grow and to establish their territory.