A nominal 6V/3W hub dynamo is "literally" to be intended as a dynamo capable to deliver 3W when connected on a 12 Ohm load, mounted on a 28" wheel running at 20 km/h.
The same dynamo is nevertheless normally capable to deliver twice the a/m nominal power at 20 km/h, i.e. 6W, if connected to a suitable load around 30 to 40 Ohm.
At this point the problem seems to be the AC/DC converter.
The whole e-werk family can't get more than 2.75W approx output at 20 km/h.
Forumslader, conversely, seems capable to do better, the only way to check if it's true is to have one and make real measurements.