While Valve has yet to actually release a proper iso for SteamOS 3 used on the Steam Deck, others have been taking it into their own hands to provide, like with the new HoloISO.
This is possible, since 99% of what SteamOS uses is open source (not the Steam client though) and so people can easily hack away at it to do whatever they want. It's not exactly the same as SteamOS 3 but it's probably the closest I've seen yet, with the main packages coming direct from Valve with "zero possible edits" the developer says.
HoloISO works with quite a lot already including the first-boot experience for the Steam Deck, the main Deck UI, the KDE Plasma Desktop Mode (including Valve's Vapor theme), Global FSR, frame limiting and more. Some issues exist though, like it not really working on NVIDIA GPUs and Intel needs some package downgrades. For AMD though, it's looking golden.
Still wouldn't exactly recommend it for full desktop use, best to really wait until Valve release it properly or at least leave it until HoloISO has had time to mature more.