I don't have a Galaxy Z Flip but the charger has worked for me with all wireless charging enabled devices. So if the Flip 3 has wireless charging, it should work. I hope this helps you.
It says that all QI certified phones are compatible:
https://www.samsung.com/de/mobile-accessories/charger-pad-p1100-ep-p1100bbegww
So you would have to find out if the A51 is.
However, it is not listed on this site:
https://www.kabellose-ladegeraete.de/blog/qi-faehige-handys-alle-smartphones-mit-qi-standard
If the phone supports wireless charging, it does. All Galaxy S models from S7 onwards support wireless charging.
Duos, as far as I know, is Samsung's term for phones with two sim card slots.
I tried this out with my Sonicare right away. The charging indicator flashes about 10 times as fast compared to when I use the original charger. The charging current seems much higher and would probably damage or even destroy the battery in the long run. The power of the Sonicare charger is rated at 1.5W for me.
I would not recommend this.