The OneUi 3.0 is an exciting update I am looking forwrd to. Currently it looks like it is only available to developers partnering with Samsung. You can read more here: https://developer.samsung.com/one-ui-beta/index.html
The public beta should be available sometime after this round and I don't know when that will be available. Usually Samsung makes the beta invite and details available in the Samsung Members app once available so make sure to keep an eye out here or sign up for any newsletters you are interested in receiving.