I have platform that is powered by 5v and consumes 26 mA.
I have a load (camera) that is powered by 3.3v and it consumes 43 mA.
If I power the camera via the 3.3v pin on the platform (which is still powered by usb), should the total current consumption be 26+43 = 69mA?
How do you reconcile the different voltages ?
Is you platform a n Arduino clone/derivative ? What 3.3V regulator does it have?
Arduino has a linear 3.3V regulator so the 43 mA it puts out will come from 43 mA (plus a little) from the 5V supply. So yes, they add.
If a system has "switch-mode" power supplies there will be less losses and the 3.3V output current COULD be more than the 5V input current.