These 2 keyboards are some of the most popular low profile 75% mechanical keyboards, with each having its pros and cons. Here is quick gist of their differences.
Price
While the Keychron might be a littles cheaper, the additional shipping fee makes the Nuphy more affordable.
Winner: Nuphy Air75
Build Quality
These tho keyboards are well build. However, the Nuphy feels more premium than the Keychron K3 keyboard.
Winner: Nuphy Air75
Switch Options
The Nuphy Air75 has 3 switch options: Red, Brown and Blue Gateron switches. The Keychron K3 Pro keyboard offers the same switches as Nuphy Air75.
Winner: No winner here.
Software
The Nuphy Air75 has proprietary software, the Nuphy Console. However, it is currently only available for Windows. The Keychron K3 Pro comes with QMK or VIA support, which is really great.
Winner: Keychron K3 Pro
Connectivity
Both keyboards support wired and bluetooth connectivity. These devices can connect to up to 3 devices via bluetooth. The Nuphy Air75 comes with a USB receiver that allows 2.4GHz wireless connection.
Winner: Nuphy Air75
Price
The Nuphy Air75 cost more, but you can argue that you get more value when choosing this keyboard.
Winner: Keychron K3 Pro
Keycaps
The both Nuphy Air75 and the Keychron K3 Pro comes with PBT keycaps .
Nuphy Air75 keycaps are close to each other and some complain about accidentally pressing unwanted keys when typing. Also since their are PBT, you don't to truly enjoy the RGB lighting. Keychron K3 Pro has floating keycaps. So the winner here is a matter of preference.
Winner: No winner here
Ergonomics
Both keyboard come with incline adjustment. The Keychron K3 Pro wins here since we have 2 built-in adjustable feet for angles of 6° and 9°. The Nuphy Air75 has magnetic feet that you can add to create an angle of 6.5º. However, It is easy to loose or misplace them.
Winner: Keychron K3 Pro
Conclusion
There is no clear winner here since it might be a matter of preference.Go for Nuphy Air75 if you wish for a more modern look, better build quality, more connectivity options. However, if you choose Keychron K3 Pro you get a more compact keyboard with QMK/VIA support and a clean looking keyboard.