You only need mobi on K1, K2, DX and DXG.
The superior azw3 works on the Kindle 3 (Keyboard) with a FW update, though the earliest FWs on the KK3 only did mobi.
All later models do azw3.
Quote:
Kindle Keyboard (3rd Generation)
3.4.3
Your serial number starts with B008 download this file.
Your serial number starts with B006 download this file.
Your serial number starts with B00A download this file.
Note: If your device is on a version older than 3.4.2, please visit Previous Software Updates for Kindle E-Reader for instructions.
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https://www.amazon.com/gp/help/custo...MQC26VQQMM8XSW
Anything not a Kindle that uses mobi will work far better with epub2.