It might not have been even released in your country yet, but the Galaxy Tab S3 is already receiving the first official update in the regions where it is available. The update is the April security patch which contains both the Android SMR for the month and Samsung's own little tweaks to cull out bugs and security gaps. The OTA package should arrive on your tablet anytime now, with the build number T825XXU1AQD6.
Given that the Galaxy Tab S3 already runs on the latest version of Android Nougat 7.0 that has yet been released by Samsung so far for any device, it isn't really a surprise that the update contains only the monthly patch. Besides, there have been no reports of the Tab S3 showing any signs of faltering anywhere to warrant a major firmware upgrade either. Although the SD 820 SoC powering the device is over a year old now, the waning market of Android tablets doesn't really have anything better to offer and therefore, it is arguably the number one Android tablet in the market, with the only other competition coming from the much older Google Pixel C.
If you are impatient about the update, feel free to try out some of the firmware packages for the Tab S3 from our firmware section.