This one is a bit unusual in that the alleged contradiction arises from an in-house dispute within the synoptic gospels, that is, between gMark and the other two. Here are all the relevant passages: KJV, NIV.
The author of Mark makes it clear that the disciples are to "be shod with sandals" and carry a staff, while the other two authors seem to have Jesus saying that the disciples can go forth without even such necessaries as these.
I would suppose that a common apologetical approach would be to say that it is understood that Jesus was talking about packing extra staves in Matt 10:10 and Luke 9:3 and that these verses must be taken in light of his earlier insistence in Mark 6:8 that they are to take just one walking stick. The only problem with this approach is that the authors of gMatt and gLuke both dropped the part about wearing sandals in their retelling of Mark's story! Such a deliberate redaction does not argue for reading the later books in light of the former, but rather for ignoring the earlier tradition in favor of the revised version of the story.