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.
Build Quality
Overall the Nuphy has better build and 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 V2 keyboard offers the same switches as Nuphy Air75, but also offers theses switches in their optical versions. That is 6 options in total for Keychron.
Winner: Keychron K3 V2
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 V2
Keycaps
The Nuphy Air75 comes with PBT keycaps, while the Keychron K3 V2 comes with shine through ABS 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 V2 have square shaped keycaps which some complained that have sharp edges and can cause discomfort when typing.
Winner: Nuphy Air75
Ergonomics
Both keyboard come with incline adjustment. The Keychron K3 V2 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.
Winner: Keychron K3 V2
Conclusion
The Nuphy Air75 seems like a better option since it has better build quality, more connectivity options. However, if you choose Keychron K3 V2 you get a cheaper and more compact keyboad that comes with more switch options.