I am not sure if certname is a fact, but you can always make some for example, lets assume all your nodes format xxxYYY where YYY is tier

Facter.add(:certname) do
  setcode do
    Puppet.settings[:certname]
  end
end
Facter.add(:tier) do
  setcode do
    certname = Facter.value(:certname)
    if certname =~ /^\w{3}(\w{3})/i
      $1
    else
      nil
    end
  end
end