See these verses fully explained in the notes at Matthew 10:9-15. In Matthew 10:5 they were commanded not to go among the Gentiles or Samaritans. Mark omits that direction, perhaps, because he was writing for the "Gentiles," and the direction might create unnecessary difficulty or offence. Perhaps he omits it also because the command was given for a temporary purpose, and was not in force at the time of his writing.

A staff only - It is likely he desired them to take only one with every two, merely for the purpose of carrying any part of their clothes on, when they should be obliged to strip them off by reason of the heat; for walking staves, or things of this kind, were forbidden, see Matthew 10:10. But, probably, no more is designed than simply to state that they must not wait to make any provision for the journey, but go off just as they were, leaving the provision necessary in the present case to the care of Divine Providence. St. James is represented in ancient paintings, as carrying a gourd bottle on a Staff across his shoulder.

(3) And commanded them that they should take nothing for [their] journey, save a staff only; no scrip, no bread, no money in [their] purse:
(3) Faithful pastors should not have their minds set even on things that are necessary for this life, if they might be a hindrance unto them, even if it is just a small hindrance.

And commanded them that they should take nothing for their journey,.... To accommodate them in it, except those things after directed to:
save a staff only; a single one, for staves in the plural number are forbidden; see Gill on Matthew 10:10;
no scrip, no bread, no money in their purse. Travellers used to put their bread, or any other sort of food into their scrips, and their money in their girdles; but the disciples were not allowed to carry either, because provision was to be made for them wherever they came at free cost, it being what their labour was worthy of; See Gill on Matthew 10:9, Matthew 10:10.

Save a staff only. Only the staff that each had already. Matthew. forbids a supply for future use.

He commanded them to take nothing for their journey - That they might be always unincumbered, free, ready for motion. Save a staff only - He that had one might take it; but he that had not was not to provide one, Matthew 10:9. Luke 9:3.