We've seen a few mentions of Android 4.4.3 up until this point. And while we're still lacking any sort of official changelog, it appears Samsung has begun testing the update on two devices. Android 4.4.3 is shown as build number KTU70 and the test devices are said to include the Galaxy S5 and the Galaxy S4 LTE-A.
Breaking this down a bit further and it looks like the Galaxy S5 testing includes the Exynos and Snapdragon variants. The Galaxy S4 LTE-A is the GT-I9506. Details about a release timeframe are just as light as the build itself. For now the update is believed to be coming before Google I/O, which takes place in June this year.
We can hope that timeline holds true, but we also have to remember this is still sitting firmly in the rumor category at the moment. Otherwise, some of those earlier mentions of Android 4.4.3 touched on new navigation buttons and a new dialer app. An earlier mention coming from the Google Edu Device setup app also suggested the 4.4.3 release was coming up quickly.
Along with Android 4.4.3 for the Galaxy S5 and Galaxy S4 LTE-A, it looks like Samsung has been testing Android 4.4.2 on a handful of devices. Similar to the release timeframe for the 4.4.3 update -- the release timeframe for the 4.4.2 update remains a mystery at this point.
For now though we did see the Samsung "KOT49H Update Status Report" which mentioned the Galaxy Note II LTE, Galaxy S3, Galaxy S3 LTE, Galaxy S4 Mini, Galaxy S4 Mini LTE, Galaxy Note 3 Neo, Galaxy Grand 2, Galaxy Mega 5.8 and Galaxy Mega 6.3.