A classic USB 3.0 host port must provide "at least" 900 mA of current. See Section 11.4.1 of USB 3.1 Specifications. Since the VBUS is common for both USB3 and USB2 data connections, a USB device (2.0 or 3.x) can practically draw "at least" 900 mA.
In general, classic Type-A USB ports don't have means to monitor the actual current consumed by downstream devices, nor discriminate between USB2 and USB3 devices. There is no architectural definitions for that. In many cases the VBUS on mainboards is "ganged" together to a +5V power rail, with resettable polyfuses between the ports. So in practice one can draw several amps out of a USB port, up to what the USB connector can carry without melting and smoking.
With USB Type-C ports the situation is a but different, up to 5 A might be available per port, with mandatory individual power switches and their built-in hardware cut-offs. The Type-C CC-signaling schema doesn't differentiate between USB2 or USB3 data connection.
Yet the situation is more different in host systems that implement USB Power Delivery. Then the port must supply only what has been negotiated in power contract, and the host is obligated to limit the supply current to a capability advertised by device. Some new devices with pretense to use PD are trying to pull out the old USB2 cheat and advertise only minimal current during PD contract; these devices usually fail on Type-C ports that implement PD, but will work on ports without PD.