We first heard about Android 4.4.3 Update for Nexus 4, Nexus 5, Nexus 7, Nexus 10 and other supported devices for a few weeks, but, the new firmware hasn't been released yet.
After the folks at Sprint announced they will release the Android 4.4.3 KitKat for their Nexus 5 variant mid-April, they backtracked saying they are facing issues with the new software. Several days later a Sony Xperia Z Ultra Google Play Edition running Android 4.4.3 visited BluetoothSIG, confirming that the new update will be rolled out to other devices, too.
Even though it was initially believed that the Android 4.4.3 will be rolled out exclusively for the Nexus 5 to address a camera bug, the most recent reports are indicating that the new firmware will also hit devices like Nexus 5, Nexus 7, Nexus 10, or Google Play Edition handsets.
New details about Android 4.4.3 come from well-known HTC developer LlabTooFer. According to a post on his blog the Android 4.4.3 update will be rolled out for the HTC One M8 Google Play Edition and One M7 Google Play Edition in May - June time frame. The 4.4.3 update schedule he got his hands on also reveal that HTC will roll out the new update for their Sense smartphones.
If LlabTooFer's information is to be believed, than the Android 4.4.3 Update for Nexus 4, Nexus 5, Nexus 7, Nexus 10 and other supported devices should be released in May - June, along with the one for HTC One M8 GPE.
The Android 4.4.3 update will be a minor update, therefore don't expect anything spectacular besides performance improvements and bug fixes. Anyway, recent leaks showed that Google may be planning to revamp the Phone Dialer app in the new update.
Unfortunately, there's no way to verify the veracity of the report, therefore we are advising you do take it with a pinch of salt.