That doesn't leave much opportunity for Luke to include bits of Mark that Matthew excluded.
Sure, there may not be as many instances as I thought, but there seem to be plenty that when analysed from the point of view of both Markan priority and Markan dependence, suggest greater likelihood that Mark was conflating the other two.
In the account of Gethsemane, Mark and Matthew are very closely parallel, and there seems to be just one instance where a clause from Luke creeps in. Luke's version of Gethsemene appears to come from a different source to Mark and Matthew, or to be a substantially re-worded version of one or other or both. At one point, Mark uses a clause that is different from Matthew but is found in Luke: "take this cup from me" (Matthew: "let this cup pass from me"). Did Luke know this phrase from Mark and use it at the same point where Matthew, having been in parallel with Mark throughout the section, departs from Mark's wording? Or did Mark, following Matthew, know the clause from Luke's account and insert it instead of Matthew's equivalent?
In some sentences, such as Mk 14:1(b), Mark has one half in common with Matthew and the other half in common with Luke:
Mk/Lk: 'And were seeking the chief priests and the scribes how'
Mk/Mt: 'by guile they might seize and kill (him/Jesus)'
Here, either Matthew and Luke pick opposite halves of Mark's sentence
, or Mark conflates his two sources.
I've been finding some other examples like this, though I've not always found them when I thought (and admittedly hoped) I might. There are also the previous type, where you get a long passage in Mark in common with with either Matthew or Luke but occasionally a clause creeping in that seems to have been taken from the other of the two.