A solid concept! Technically it feels a bit more like 'change is recommended' than is necessary, I was able to get through the course without needing to use the transformation platforms after a few tries. :)
I do like the idea, getting a denser form to push things, a lighter form to be lifted into the air and the base form to... well it doesn't seem like the base form has direct advantages over the other two? It might actually be better if there were just a light and heavy form to swap between. The player could start as the light form which can't push the boulders at all, switch to heavy form for that, then need to switch again for the updraft near the end.
Anyway, it's a good start for a game as it is! It's a shame you didn't have time to implement more levels but that's the way game jams go.