series charging three 18650 batteries with three chargers off the same 5V supply
No, this won't work. You can't simply daisy chain chargers but drive them off the same 5V supply.

Your charger needs to be able to define both the out+ and out- rails (chances are, out- is directly connected to in-, anyway). Same goes for b+ and b-.

So, by doing this, you're shorting battery + to 5V -. That's a way to start a fire.