Mysterious monoliths really are like buses, you wait millions of years for one and then three show up all at once.
Last week, one showed up in Utah. Before you could even load the "I'm not saying it's aliens but it's aliens" GIF, a second one showed up in Romania, and the first one was taken down. Now a third has shown up in California. At some point, we're going to have to explore the possibility that aliens are fans of whack-a-mole.
People have of course been trying to line them up geographically to see if they can glean any information. It'll probably eventually spell out "Buy Pepsi" or some other marketing slogan, but for now (this one's for you, math fans) it just makes a triangle. What did you expect?

From this, we can make an educated guess that they're drawing an "I'm with stupid" arrow pointing at Moldova.
There are no new clues as to who put up the new monolith (yes, yes, monoliths are technically stone but we refuse to call it a big rectangle metal dealy and get ourselves bullied by all the other media outlets) nor whether it is connected to the other two. It's likely the first one was left and forgotten about back in 2015/2016 when it was placed there, whereas the other two being placed extremely recently implies they are copycats. It would be weird and remarkable if they all went undiscovered for years only to be found in one week, so let's Occam's Razor this and say the latter two are copycats until proven otherwise.
As well as the monoliths popping up like Pret a Mangers in London, new footage has emerged of the dismantling of the first monolith, which was discovered in the desert in Utah. Witnesses saw the team of people who removed it and a message left in the sand. The new video, posted (of course) by a YouTuber, shows the team walking away with the monolith.
"On the night of November 27, 2020, at about 8:30 pm— our team removed the Utah Monolith," user MrSlackline wrote on YouTube. "We will not be including any other information, answers, or insight at this time."
In the footage, you can see inside the monolith, which appears incredibly human-like in origin, and could easily be made by humans. It was dismantled pretty easily within minutes, so either it is human-made (it is) or we're looking at the sloppiest alien work in the galaxy, announcing their presence to another civilization with a "screw it, that'll do" attitude.
Inside the monolith, there appears to be.... nothing. No inscription, no second smaller monolith, and no alien sat on a toilet. As for why the monolith was put up in the first place, that has likely already been solved. Until the next one pops up on Jupiter and sends us hurtling through a wormhole before making a massive star baby in space (I realize this sentence may seem odd to anyone who hasn't seen 2001: A Space Odyssey), it's safe to assume what we're looking at here is a bunch of hunks of metal and a few hiking trips by pranksters.