Hi,
I did see one card online at the store I usually buy from. It's a Radeon™ Pro WX 2100 for $200CAD that supports up to 3 4K monitors. This isn't a recommendation because I haven't done any research or comparisons, but you might be able to find something.
Looks like about $140 USD on amazon here - but only 2GB RAM? Won't that affect the maximum resolution?
No. The actual display data for 3 4K monitors is less than 100MB. The rest of the memory is used for buffers and 3D rendering stuff. Even if you don't play games, the 3D rendering hardware is used by compositors like Gnome Shell and video playback.
Am I going to notice a big difference between a 2GB card and a 4GB card if I don't use it to play games?
I believe the card I have now only has 1GB (it may be more than ten years old, lol), but I would like to invest in something that's going to last a while...