First off a correction: All 3 synoptics do mention the 3 hours of darkness. However, Mark and Luke say nothing about earthquakes or tombs being opened. They instead talk of a tapestry in The Temple getting spontaneously torn1. Supernatural perhaps, but kind of weaksauce in comparison.
The general understanding of the synoptics is that Mark was written first, and the other two used Mark as a source. Matthew was particularly obvious about it, as it contains more than 90% of Mark. Of course Matthew and Luke modified those verses a bit, to suit their own purposes, and added a bunch of their own new stuff (some of which was common between the two, leading scholars to postulate a lost common source).
There are a couple of important implications of this for the purposes of this question:
- For anything found in all synoptics but not John, its more reasonable to consider that as having come from one source, not three.
- For anything found in all synoptics, but with extra details in Matthew or Luke, its most reasonable to consider that these were details added to the original Mark story at a later date. The fact that Matthew and Luke also don't match is further evidence.2 The (apocryphal) Gospel of Paul took the process even further, talking about all of Israel being affected, and people having to walk around with lamps.
There's also a Middle-Eastern literary tradition of major supernatural events happening at the moment of a major figure's death. Logically, if you're writing something arguing that Jesus was such a figure, this is the kind of detail the writers would want to add if it didn't exist already. The story would have looked weaker to a Jew without it.
As for what could have happened without getting recorded, well you're right that its possible, but I could say that about literally anything. Aliens from Alpha Centauri could have been involved, but no sources including the Centaruians survived. There are so many possibilities here, that reasoning about events from this end is completely useless to us. Logic of that kind is what's often called Not Even Wrong.
As historians we have ways to reason about history with our limited knowledge. In fact, this exact kind of reasoning about the historicity of The Bible was early on one of the whetstones we sharpened our Historical analysis tools on.
So from a historical perspective, the best way to view The Crucifixion Darkness is that its not up in the list of first-class historical events of Jesus' life3. Within where it does exist, the darkness itself is better documented than Matthew's unique earthquakes, and slightly better than the torn temple tapestry (which Luke felt free to tinker with, and Matthew didn't bother to mention).
1 - It seems like this was meant to be referring to the Temple Veil that separated the general public from the Holy of Holies, where only the High Priest was allowed, and then only at Yom Kippur. To my mind the metaphorical meaning of this bit is so clear that historicity of it is totally beside the point. You may disagree with that assessment, but it appears Matthew and Luke did not.
2 - Luke merely edited Matthew to have the Temple incident happen before Jesus' death, rather than at it. If you are a literalist, you might have a problem here, because one of the two has to be incorrect, barring some extreme mental gymnastics (like 2 different separately torn Temple tapestries that neither found interesting enough to mention the other of)
3 - or even in the second class of arguably likely historical elements.
