It looks like Samsung is already testing Android 4.4.3 on its current flagship, the Galaxy S5, and last year's Galaxy S4. Documents obtained by SamMobile reveal that the South Korean manufacturer is testing the as-yet unannounced Android update (which has a build number KTU70) on both the Snapdragon as well as the Exynos-powered variants of the Galaxy S5 in addition to the LTE-A variant of the Galaxy S4 (GT-I9506A).
Along with the Android 4.4.3 update, the document reveals the timeline for when Android 4.4.2 KitKat will be available on other devices. The update is listed as being in the final stages of testing and is scheduled to roll out to the Galaxy Grand 2 and Galaxy Mega in the month of June. Android 4.4.2 has begun rolling out for the Galaxy S3 LTE (GT-I9305) earlier this month, while the document suggests that the Galaxy Note 3 Neo will receive the update later this month.