I have a 5 V 10 Amp relay module and a Raspberry Pi 3. I am using it to connect to a submersible pump which runs on AC power. Can I use the 5 V GPIO pin of the Raspberry Pi or do I need to buy a DC adaptor?
This might help. You should stay away from the RPi with relais and so on from a power point of view. The RPi is not made to provide power for those devices. The RPi is only capable of controlling them the way I describe in the above mentioned link.
I'm not sure you have a clear understanding of what's going on. Or perhaps I can't read what you wrote.
You need an AC->DC converter. This is not negotiable. Whatever you give power to the Pi through the USB or the +5V pin changes little. Decent industrial 220VAC -> 5VDC converters for this power can be as little as 10 bucks. 220VAC to pi = BOOM.
You will need a supporting relay board.