Optiplex 3000 Micro PC HDMI 2.0b Option Card

Here is the optional HDMI card for your system.

https://www.dell.com/en-us/shop/dell-daughterboard-i-o-hdmi-card/apd/cpx-3v9n2/controller-cards

@Chino de Oro​ Many thanks for the part number and link, unfortunately Dell UK do not sell this part, and Dell US don't seem to ship to the UK.
There are also noine currently available on eBay in the UK.
Do you know if there are any other HDMI Option cards compatible with the OptiPlex 3000 that I may have a better chance of obtaining? (I've seen some 01KNYY and H6HFK cards here)?

You can source from Amazon UK here  https://www.amazon.co.uk/Deal4GO-Daughterboard-WX01J-Replacement-Precision-Green/dp/B0DJ6NPX61

Or, you can get USB type C from Dell  https://www.dell.com/en-uk/shop/dell-type-c-i-o-daughterboard/apd/cpl-gmhy5/controller-cards.  Contact Dell Spare Parts UK and verify if that USB-C daughterboard support UHD video when connecting to monitor using a USB-C to HDMI adapter cable.

Just an update on this, I took a chance on an eBay item the Chinese seller advertised as: "New For HDMI2.0b HDMI IO Daughter board For Dell OptiPlex 7070 7080 7000MFF" with the MPN of 1KNYY HXPK2 01KNYY 0HXPK2 - for only about 30 USD delivered to the UK.

Item actually sent was marked as CN-054YMM, also supposedly compatible with many Optiplex's - now fitted and working fine drivig my HDMI 2.0 cable connected TV at 3840 x 2160 60Hz.

Looks like multiple Dell part numbers work on this PC.