Are 3-mana rocks still used?

[anjinsan]

I actually really like the borderposts, and feel they do have some uses, but they're really... under-pushed. As rocks, they're not just cmc 3 they're particularly bad. That they can function kind like bouncelands is nice, though; they essentially act as basics, then, which is valuable if you're missing your drops. I wonder if you could almost just replace lands with them, but I don't think the upside is worth it.

I would consider them for Ramos, Niv-Mizzet, Reborn... maybe General FR? But a 3-mana rock needs a big upside, and these are particularly bad rocks.

Midnight Clock, Cursed Mirror and The Celestus I like a lot.

I sometimes also play "big" rocks like Gilded Lotus or Thran Dynamo. These fill a different role to 2-mana ramp, though. Likewise I think there's a space for 3-mana rocks as general value pieces but probably in addition to a base of better/faster ramp.

[Mynus]

Looks like I am still running Worn Powerstone (I know it enters tapped, but the 2 mana is attractive to me for some reason) and Mana Geode for the scry.

[Schau]

Looks like I am still running Worn Powerstone (I know it enters tapped, but the 2 mana is attractive to me for some reason) and Mana Geode for the scry.

I use worn powerstone a lot, it just feels good in deck with a commander that costs 6 mana so you can play it turn 4.

[robort]

The 3 mana rocks are slowly going to the wayside. With so many cheap 2 drop rocks out there that 3 drops aren't even worth it. Even commander's sphere that is put into the pre-cons I'll cut from the deck just to put something cheaper in. Even a Fractured Powerstone gives you a colorless mana. Even Mitch's favorite Wayfarer's Bauble gained popularity over 3 drop mana rocks. It just some people have adapted to the speed of the game and use the 2 drops over the 3 drops even in casual settings.

[UrizenII]

TL;DR: Yes, but only situationally.  If you have access to green, play a fast deck, and/or have a commander that costs 3 or less, it's probably not in your best interest to run them.

I'll echo what everyone else has said in that it depends on how many colors you run in the deck, but what I'm seeing overlooked is that it depends on the CMC of your commander and how fast the deck is.  For example, if your commander has a high CMC - say five or more - then I'd consider running some regardless of how many colors the deck is - especially in decks that don't have green and consequently have fewer reliable ramp sources.  However, if your commander costs three or less, I probably wouldn't run them.  Ideally if your commander costs three, you're tyring to consistently cast it on turn three and not a 3-cost rock.  If your commander costs 1 or 2 and you're casting a rock turn three instead of spells that further your gameplan (or at least cheaper/more efficient ramp), you're already falling behind.  Even in a four or five-color deck, I'd sooner run things like Fellwar Stone or Ornithopter of Paradise than a Commander's Sphere.  If that's not enough, I'd still look at the Talismans first.  In my Breya deck, my mana cost distribution is 45% blue spells with none of the other colors being higher than 21%.  I just run the three Talismans that produce blue and one of my other colors instead of running 3-cost rocks.

Chromatic Lantern is probably the most worthwile 3-mana rock to run since it perfectly fixes all your colors and can itself tap for one mana of any color, but only in non-green three-color decks and four or five-color decks.  I think Coalition Relic is grossly underrated in this category, as it can get you two additional mana of any color the turn after you cast it as opposed to only one (tap for a charge counter, untap at beginning of next turn, then tap for another charge counter prior to precombat main phase).  Skyclave Relic is a strictly better Darksteel Ingot because it has the kicker option.  These three are really the only ones I'd consider using universally (and yes, Commander's Sphere is not in that list).  There are others that are useful, but only in certain decks (i.e. Altar of the Lost in Flashback decks, Basalt Monolith as a combo piece, Dragon's Hoard in Dragon tribal decks, Elementalist's Palette in X-spell decks, Strixhaven Stadium in go-wide combat-focused decks, etc.).

[anjinsan]

There are definitely 3-mana rocks that fit specific decks. That includes Commander's Sphere (e.g. combo with Sun Titan). I also think people are a bit unfair to Chromatic Lantern as fixing is important and it's one extra way to do that (but if you have green, which is likely at 4+ colours, you probably have better ways).

However in general, nope. I'd rather use Wayfarer's Bauble (in non green) because even though the raw rate (3 mana, enters tapped) is poor, diversifying your ramp is good (basic lands won't be Vandalblasted) and the split cost means you can do it T2 still. That and I have to make a special effort not to use Salvaging Station in every deck, these days :-)

[Serberus_08]

I only really use Chromatic Lantern for colour fixing tbh. Sometimes commander's sphere if I can play Sun Titan or a way to bring it back to play.