UPDATE:
Crashed and broke one of the original motor due to a desync caused by wrong Dynamic Idle value in Betaflight. I've replaced it with Mamba 1408 4000kv motors and converted to Emuflight at the same time. The flight characteristic of this 3" is amazing after relatively ease of tuning in Emuflight. I've shared one of my recent flight here:

I found this budget frame ($16) on Banggood and bought this mainly because of the design - it looks just like my favourite QAV-S 5" shrinked proportionally to a 3".

This is the most customized and experimental build so far for me. I had to modify the frame (by sanding the front camera mount and drilling hole on the top & bottom plates to shift the rear standoffs) to fit in the Jhemcu AIO which is a longer version of 20x20 board and stack the naked Caddx Vista on top all in the middle of the frame - the end result is a very satisfactory fitting with ample room for buzzer, capacitor and RX at the rear of the frame.

I have also adopted the Happymodel ExpressLRS 2.4g TX and RX for the first time and finally realised what it mean to have minimal latency - the control of this quad is even better than my 5" with my initial tuning.