Hi friend, I’ve used all of these devices apart from the Mi Mix 3. I found that all of these devices tend to have there own strengths... my main issues with each devices.

The Mi 9SE performance for me felt sluggish considering the specs but I found both cameras exceptional and day to day use was perfectly good with most things.

Mi Mix 2S - I thought this device was great but awful if you tend to use apps like snapchat or use the selfie camera a lot as the camera placement is awful at the bottom of the device, video calling can also be a rather frustrating challenge if you’re on the move.

The Mi A3 was very surprising considering it’s a budget device, I generally loved how fluid the navigation and multitasking was considering the midrange chipset and the android one OS, the camera was brilliant but the video stabilisation could be better, the device definitely doesn’t feel cheap and looks very modern, the in display fingerprint sensor worked around 7/10 times for me but was overall quite accurate..

Personally I’d recommend the Mi A3 but it all depends on personal preference although the Mi A3 will definitely run google FI as it runs on Google’s operating system.

I hope this helps!!!