Fascinating assortment of lands in this set. This one is very interesting, but I fear it's too powerful. Tapping repeatedly for 3 mana is just crazy, and you could typically activate this 3+ times a game. Getting 3 mana from a single land should automatically make it rare, and almost automatically mean the design just gets killed or changed. Even if you added "Pay 5 life" to the cost, this might still be too powerful. It's a Turn 1 Phyrexian Negator, for example, or a Putrid Leech with a blue mana up. Two of them mean a Turn 2 Grave Titan/Primeval Titan/Wurmcoil Engine!

Also, suppose my mana-base consists of 4 of these and 20 basic lands. I probably don't care about the fact I can activate it "only" 9 times or so, and in addition I'm thinning my deck; making me more likely to draw spells rather than lands once I've got my mana up.

That's a bit more reasonable. That's now quite interesting, somewhat comparable to Gemstone Mine or Murmuring Bosk. It helps you towards threshold, but there's very little that can actually benefit from lands in the graveyard. Terravore, Lord of Extinction, maybe Groundskeeper. Nothing too broken.

It is still thinning your deck; it'd want playtesting, but I suspect it'd be better balanced with "~ ETBs tapped" as well. But nonetheless, a much more sensible design.