I didn't test but this should work:

import-csv users.csv | foreach {

    $mailbox = get-mailbox $_displayName

    $emails = $_ | foreach {$_.proxyaddress,$_.proxyaddress2,$_.proxyaddress3} | where {$_}
    if($emails)
    {
     $mailbox.emailAddresses+=$emails
     set-mailbox $mailbox -emailAddresses $emails
    }
   
    set-mailbox $mailbox -primaryEmailAddress $_.email
}