Some minutes ago, I had a short test with a Shelly 2.5 powered by 3,3V DC:
Voltage at terminals SW1 and SW2 was 2,4V DC (compared to 3,1V DC @12V DC supply voltage).
Input current at SW1/2 is -26μA (compared with 34μA @12V supply).
Threshold is approximately 1,1V (compared with 1,6V @12V supply).
Reference potential is terminal L/GND.
Activating the inputs in relay mode is possible with all button types, but is acting very slowly: Button must be pressed for approximately 1 second to get a signal at WebUI. Button type „detached“ seems to work best…
Activation in roller shutter mode was also tested: Unfortunately there is no optical signal at WebUI in roller shutter mode if inputs are „detached“. But commands are given in one or two button mode. Long button pressure is necessary like in relay mode.
After this test I would recommend to power the Shelly 2.5 with 12V DC to assure reliable operation. May be a modified Shelly i3 could save resources (it is cheaper than a Shelly 2.5).