Back home and applied some google-fu...
SMD-codes can be tricky and most codebooks aren't nearly complete. If they don't help you have to turn to the datasheets from the manufacturers. But that means knowing "who" and "what" first.
Thanks to
AliExpress, which lists the marking (code) as well, I found the most likely candidate is the
Torex XC62FP-series.
Turns out the marking scheme on the ICs seems to fit the specs in the datasheet perfectly:
The 3A0X marking confirms:
3: voltage of 3.something Volts
A: the "something" is .0, so "3A" = 3.0 V;
0: fixed, but probably unique for Torex;
X: changes with every lot, so doesn't mean anything to us.
Since the board has "3 V" printed next to the pins of some headers, I was assuming from the start that U2 and U21 were most likely voltage regulators of 3 V. This together with the above data means I'm reasonably sure that U2 and U21 are Torex XC62FP3002PR, but I can't guarantee it. You may try these, but you do so at your own risk.