A group portrait of three is still small and intimate. It lends itself well to a pyramid- or diamond-shaped composition, or an inverted triangle, all of which are pleasing to the eye.
Don’t simply adjust the height of the faces so that each is at a different level; turn the shoulders of those at either end of the group in toward the central person as a means of looping the group together. You can also try creating a diagonal line with the faces at different heights and all the people in the group touching.
Or, create a bird’s-eye view—cluster the group together, grab a stepladder or other high vantage point, and you’ve got a lovely arrangement. It’s what photographer Norman Phillips calls “a bouquet.”