I would like to know if I can set my flash on auto in Aperture priority mode. I have a D3200 Nikon. And also, can I set auto ISO in Aperture priority mode?
1) Yes & No. See pages 47-50 of your D3200 User Manual. You can select Fill flash, Red-eye reduction, Slow sync + red-eye, Slow sync, and Rear-curtain + slow sync while in A mode. All of these modes can automatically calculate the amount of flash power used to correctly expose the shot. The Auto (auto flash) mode that will determine if you need flash and pop up on its own is not available in A mode.
2) No & Yes. See pages 51 and 52 of the D3200 User Manual. If ISO is set to Auto and the mode dial is turned to any of the PASM modes, the camera will return to the last ISO setting selected when it was last in that particular mode. If the chosen ISO and aperture will result in underexposure at the minimum shutter speed selected (p.50 - the minimum shutter speed changes depending on which flash mode is selected), then Auto ISO (if enabled) will adjust the ISO to obtain correct exposure. From page 129 of the D3200 User Manual:
When On is chosen, ISO sensitivity will automatically be adjusted if optimal exposure can not be achieved at the value selected by the user (ISO sensitivity is adjusted appropriately when the flash is used). The maximum value for auto ISO sensitivity can be selected using the Maximum sensitivity option in the Auto ISO sensitivity control menu (choose lower values to prevent noise (randomly-spaced bright pixels, fog, or lines; the minimum value for auto ISO sensitivity is automatically set to ISO 100)). In modes P and A, sensitivity will only be adjusted if underexposure would result at the shutter speed selected for Minimum shutter speed (1/2000 – 1s, or Auto)