Wow. This seems much, much better than Darksteel Ingot and the rest of the Manalith family (well, except Chromatic Lantern). Also much better than Sphere of the Suns and friends. I like the idea of a green-phyrexian mana rock, but this seems rather over-pushed at the moment.

There several 3 CMC mana rocks that do a good bit more than add one of any color:
­Coalition Relic
­Commander's Sphere (in Commander)
­Cultivator's Caravan

Sphere of the Suns has a kinda low power level, so I'm fine with this generally outclassing it. However, it is worth noting that having to pay 2 life or a green mana, while it isn't much, isn't nothing.

Also also, in EDH, this can only be played in green decks, which already have access to better ramp.

I do see how it is pretty pushed, but I don't think it would break any format or anything.

Doesn't actually break anything. But is a "Run 4 of" in every deck, regardless of colour.

Which is a bit of a no-no, sadly. I admit that I have a weakness for CMC2 mana ramp.

I think it entering tapped would be the most sensible fix: Charcoal Diamond and friends + Coldsteel Heart.

There are lots and lots of 3-CMC mana rocks that are Manalith++. Astral Cornucopia, Spectral Searchlight, Vessel of Endless Rest, in addition to the ones you mentioned. (Which makes it all the more entertaining that there are so many old cards worse than Manalith as well: Obelisk of Bant and its friends, which are better than Seashell Cameo and friends; Phyrexian Lens; and Mana Prism, which is better than Standing Stones and Celestial Prism. Early mana rocks were really bad.)

I agree with Tahazzar that ETBT is the thing to do here.