Samsung launched the obscenely expensive Galaxy Tab S3 last spring with Android 7.0 Nougat, and it's been on Nougat until just recently. The Oreo OTA started hitting devices in the UK several weeks back, and now it's time for the US market. If you hit the update menu right now, you'll find an OTA waiting.
The Tab S3 update is based on Android 8.0 Oreo and has Samsung Experience 9.0. That's the same version that ships on the Galaxy S9. When the OTA is complete, you'll have all the usual Oreo features like autofill apps, picture-in-picture video, and more. Samsung's custom features will also get some tweaks with a new home screen, updated keyboard, and the new SmartThings app.
The update clocks in at about 1.2GB, so give it some time to download and install. You can also choose to have it automatically install overnight. You should make sure to back up your data just in case something unforeseen happens.
The update should also be hitting Verizon's variant of the Galaxy Tab S3. It has build number R16NW.T827VVRU1BRD4 and comes with the April security patch.