Does an ac plugged in macbook provide extra power to unpowered usb 3 hub?

A MacBook Pro with four TB3 ports can power two external devices at up to 15 W and two external devices at up to 7.5 W; a MacBook Pro with two TB3 ports can power one external device at up to 15 W and one external device at up to 7.5 W. It doesn't matter which of the ports are used, power is provided first-connected first-served.

It doesn't matter if the Mac is connected to AC power or not in terms of the power delivered (however, if the Mac is running on battery it will obviously only be able to deliver that power as long as the battery lasts).

The above applies to TB3 ports, and it's 'shared' in the sense that, for example, a total of 45 W is available on a 4 TB3 port Mac, so you can plug a 15 W peripheral into any of the 4 ports and it will get 15 W, then you can plug a 10 W peripheral into any of the remaining 3 ports, and it will get 10 W...but then you've used your two 'up to 15 W' devices, and if you plug in a second 10 W device it will only get 7.5 W. Hope that makes sense.

In your case, your USB 3 Mac will deliver up to 900 mA at 5 V to each of the two ports. So, if you have two HDDs that each draw 4.5 W (900 mA at 5W), you can plug them both in and they'll both work.