For 3 or less SGs with no other fish, it would be fine. Trust, I'm the last person to suggest someone keep a fish in tight quarters, but these really are a shy and sedentary fish. Three short-finned guppy in a 5 gallon would be terrible, they are much too active and need more room. But not only could
3 Sparkling Gourami live in a heavily planted 5g, but they'd be happy to do so.