Greetings... as per topic subject, pretty much.

I have my old saves from the original SimCity 3000 on a pendrive, and before buying SimCity 3000 Unlimited (since SimCity 3000 isn't available) I wanted to make sure my old cities from the original game are compatible/can be imported into SimCity 3000 Unlimited.

Any insight would be much appreciated, thanks!

I don't see why not. I would have shown you an old SC3000 archive, but such the problem is that the archived site appears to have been eaten by Simtropolis in the interim.

So instead, here's the web archive.

even SimCity 2000 cities can be loaded into SimCity 3000 Unlimited, so there's definitely no issue from earlier SimCity 3000 versions

it actually is technically possible to take an original SimCity classic save forward to SimCity 4