Background checks on all tennants or just the primary?

[Fred Krueger]

Sorry if I'm reposting a question, I couldn't find the answer anywhere. Found tennants for my three BR home. Three unrelated people. Should I run a background check on all of them? And collect the background check fee from each of them? They will all be on the lease. Or should there be just one primary applicant with two co-applicants and only one background check? God I'm such a newbie!!!