Hi!

I have a pi3 too with a 2.5A power supply (previously used a 1A one) and what I felt is that just the pi itself uses around 0.6-0.8A when used. I'm not sure about which USB miner you want but you only have around 1Amps from a 2A power supply. Also please note that the voltage will drop when you're putting load onto the charger. I'd recommend looking for a miner that eats 1A at max.