I'm using a USB 7 port hub that's rated 5V at 3.0A and get a lightning bolt symbol in Raspbian. There's nothing else plugged into it.
Shouldn't this work?
Your PSU isn't powerful enough. Try the official Raspberry Pi PSU. Also try plugging in a power adaptor to your USB hub if it has a jack.
Lightning bolt means undervoltage.
Measure the voltage you're getting out of your hub (with a multimeter, USB charge monitor, whatever) and on the power pins of your RPi. Either your hub is not supplying a high enough voltage, or your cable is bad enough to drop the voltage on the RPi side.
Also, a "3A" label doesn't magically turn a bad power supply into a good one.