If you read the passages in parallel Christine you will see that Mark is very often more detailed than both Matthew and Luke when discussing the same passage. You then have to explain why both Matthew and Luke copied Mark but both left out his detail. If you like I will go and dig up my compared passages from a few years ago so you can explain how it makes sense for Matthew and Luke to independently both leave out his extra detail.
Let’s review the data we have. I’m gonna get wordy here again, so pardon me.
Triple tradition: Refers to the material shared by Matthew, Mark and Luke. It broadly has the same order across all three gospels, in fact this order tends to be identical with Mark’s, to the point that if you were to isolate triple tradition material in Matthew and Luke you’d end up with a complete gospel generally similar to Mark in structure. Now there are times when Matthew or Luke may occasionally place individual incidents differently, but striking thing about it is that it is rare for both Matthew and Luke to place the same incident differently. Even where Matthew and Luke apparently depart from Mark’s narrative order, they very often both of them end up reverting into agreement with Mark.
Hence, one of its characteristics of triple tradition is that it has a lot to do with Mark, i.e. Mark is often (but not always) the mediating factor or the so-called ‘middle term’.
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Double Tradition: Material found in Matthew and Luke but not in Mark. The order tends to vary between the two gospels. This is what supporters of the two-source hypothesis call ‘Q Material’. It is not as numerous as triple tradition material (it’s roughly half the length of triple tradition), but is still substantial to some extent. The interesting thing about this material is that you don’t have much of it in a closely parallel order; there is some kind of parallel order, but not the same one you get with triple tradition. The order tends to vary between the two gospels.
Special Matthew (M): Material found in Matthew alone. Like double tradition, much of it is sayings material (for instance, the parables in Matthew 25:1-13 and 25:31-46), with a few exceptions (i.e. the temple tax in Matthew 17:24-27). Some of it can also be found embedded within triple tradition material; for instance, Judas’ death (27:3-10) and the brief reference to Pilate’s wife (27:19).
Special Luke (L): Material found in Luke alone. It’s usually narrative material like the announcement to Zechariah and John the Baptist’s birth, the Annunciation and Visitation, the boy Jesus in the temple and the Road to Emmaus, and also sayings material like the parables of the Good Samaritan and the Prodigal Son.
Those are the basics. Here’s the variations to the rule:
M in Triple Tradition: Material unique to Matthew embedded in triple tradition material and would make no sense outside of context; for example, Jesus’ conversation with John the Baptist just before His baptism in 3:14-15.
Lukan Triple Tradition: Three stories which have parallels in Matthew and Mark and might be described as Lukan versions of triple tradition material (the rejection at Nazareth at 4:16-30; the call of the first disciples at 5:1-11; the anointing of Jesus at 7:36-50).
Not Quite Triple Tradition: Material found in Matthew and Mark but not in Luke or in Mark and Luke but not in Matthew. These are not, strictly speaking, triple tradition material since they occur in only two gospels, but they are akin to triple tradition because they appear in the Markan order.
Mark being not the middle term: Some material halfway between triple and double tradition. Appears in all three synoptics but unlike triple tradition, features substantial agreement between Matthew and Luke against Mark.
Mark being the ‘middle term’ is an important factor here. There are really two ways you can explain this phenomenon: either Mark was written
first, with Matthew and Luke making use of him (as per the two-source theory and the Farrer-Goulder theory) or Mark was written
last, with him combining Matthew and Luke (as per the Griesbach theory).