See Weave of Boughs.

A triple-land that doesn't ETB tapped; is hexproof and might at some point become a stompy creature?

Ok, it's legendary - but you just obsoleted [Taiga. Which is a rather courageous move, minister.

This is not better than Taiga. It has no basic land types, which is Taiga's huge strength. I do think it's probably too strong, though.

I was going to say it doesn't need to enter tapped, since it has summoning sickness, but then I realized that it won't be a creature early game. It should really not make mana early, but late game game any color the turn you play it is fine.

I imagine this being released in a Commander supplemental product. In Commander in would be a lot like Command Tower+, which is already an awesome card for Commander, I suppose, but useless elsewhere. This isn't useless elsewhere but would only be legal in Legacy and Vintage. Wizards has shown a willingness to push the power level of new cards in supplemental products, but I don't know much about Legacy and Vintage myself. How powerful would this be in those formats?
Also, how strong would it be in Commander itself? It avoids the Commander tax, sort of like Derevi, Empyrial Tactician, but does it better in some ways by never costing any mana at all. It fixes your mana starting turn one and allows a three-color deck. But once it's a creature, it's not that powerful, as far as other commanders go. It's nearly a vanilla 3/6. Obviously it's a 3/6 you got for free, though. Should I bring it down to a 1/4 or is the general concept just unacceptably strong, even for only eternal formats?

I think this would be fine in Commander. Sure, it's a free creature, but only once there are enough other things around that it's likely to be easy to block (a single Hill Giant will do). Taking the power down to 2 is probably sensible so that it's not annoyingly persistent with something like Whispersilk Cloak. The big difference compared to Derevi or whatever is that, in exchange for getting a guaranteed Command Tower on turn 1, you don't have a powerful Legend's ability available. You don't have the creature-cloning of Riku of Two Reflections, or the guaranteed stompy card-advantage dragon of Intet, the Dreamer, or what-have-you. I think that's quite interesting.

As for eternal formats: I don't see it being much of a problem, because people don't have many creatures in Legacy/Vintage in general. It's a very good land; you could perhaps argue it should have ETBT; but I don't think an untapped RGU land would break Vintage. Being Legendary many decks would rather only play one.

Yeah, he would have some shortcomings as a Commander in terms of being able to do cool things.