something like this should work:
"Mailbox01","Mailbox02","Mailbox03" | % { Get-MailboxPermission -Identity $_ }
Have to use a foreach because Get-MailboxPermission
doesn't accept [string[]]
as pipeline input or you can do:
"Mailbox01","Mailbox02","Mailbox03" | get-mailbox | Get-MailboxPermission