First, those 6V/3W are specified with a constant 12 Ohm load at 15 km/h (~9 mph), second your forumloader is a lot of things, but not a constant load. To my knowledge there are no measurements dealing with dyno hub efficencies when the hub is coupled with a usb charger. But in a (german) bicycle magazine (fahrradzukunft.de), the SON28 was measured with a 24 Ohm load (6W), giving peak efficiencies of 78% at 10 km/h and 70% and 30 km/h (~19 mph). Usually dyno hubs are more efficient at higher loads, and since the forumslader has an IC, which adapts the load to the speed, you probably have at least 75% efficiency at all speeds. So you'll end up with 13-14W mechanical power required for your 10 W, which is still less than the drag induced by a lousy 3 W sidewall dyno (>15W at 20 mph)...
Source https://fahrradzukunft.de/14/neue-nabendynamos-im-test/ (see graphs 17 and 18 and the paragraph below those graphs)