Have you had this trouble before? Because I almost always use some sort of finings (Irish Moss) and have never had trouble with bottle conditioning after a secondary. I'm not experienced with gelatin, but I would imagine that there would still be plenty of yeast suspended to carb the bottle within 3 weeks. If you are that concerned, I don't see why adding some yeast before bottling would be a bad thing.
First State Brewers
Primary: StrawberryHop Saison
Bottle: Rochfort 10 Clone, Bourbon Oak Pumpkin Ale
Keg: Kolsh, Marzen, Saison, Belgian Strong Ale, Brewskie the Elder,Pomegranate Apfelwien, Rye Dortmunder Export, English Bitter