On Sat, 12 Dec 2020 14:15:53 +0100
Antony Stone <Antony.Stone@???> wrote:

> Hi.
>
> I've just installed a couple of Beowulf systems, each of which has
> three ethernet interfaces; one on the motherboard, and two on a PCI
> card.
>
> I'm trying to work out how to give those interfaces the names I want;
> the motherboard as eth0, and the PCI card as eth1 / eth2.

Err, what is not clear in 'man interfaces'
FWIW, I've just had to do a fresh install of Beowulf and as I always do
I manuay define /etc/network/interfaces, e.g.

# The primary network interface
allow-hotplug eth0
iface eth0 inet static
        address 192.168.x.y
        netmask 255.255.0.0
        gateway 192.168.x.z

Just add the line
    hwaddress mac:add:dress:in:usual:format

to match eth2 & eth3 to the desired card.