Fortigate system DNS limited to only primary and secondary?

Hi All,

Is there a good way to specify more than 2 DNS servers at the FortiGate system level?  It looks like all you can do is:

  config system dns     set primary     set secondary

I really need to be able to add a third (and maybe fourth) dns server.

I realise that one option would be to edit the DHCP server settings for every interface, as below, but I want DNS requests to always go through the FGT's IP (as that lets me block DNS to bad actors):

  config system dhcp server

    edit <n>       set dns-service specify       set dns-server1 DNS-IP1       set dns-server2 DNS-IP2       set dns-server3 DNS-IP3

Some posts say you can set multiple DNS IPs for a single field by separating them with a comma.  

That doesn't seem to work in current versions.

Any suggestions?