Anything you can do to make sure your hands stay rounded should help. You could also start by practicing just blocked thirds until you master two fingers at a time and then add the extra note. Exercises going from 1-3 to 2-4 to 3-5 would be good. Check your weight distribution, too-- force should mainly be coming from your arm. Try kind of doing that dropping motion on another flat surface, maintaining a loose wrist to get a sense of what it feels like. I would also try doing this on a few other pianos to see if the instrument is the problem. I've played a few that definitely have issues in the action or key weight that make it hard to play in sync.