If you are planning a one-way rental (it's not clear from your question), in my recent experience (April 2023) I could not find any car hire firm prepared to offer a one-way from Italy to any of Austria, Slovenia, Croatia. No idea why. Hertz Slovenia were happy to offer one-way to Austria or Slovenia.
The actual border crossings will be embarrassingly easy, compared (say) to the rigmarole to be found at any UK border... Indeed, once (accidentally) I drove Slovenia - Italy - Austria and back again without actually having my passport with me at all - but it was fine because no one cared to check.
edit I forgot to mention, although you may well already be aware, that use of highways (motorways) in Slovenia requires an e-vignette, essentially an electronic toll pass. You can buy this online once you know your hire car's registration number. Italy and Croatia have no such payment required.