When I want to create corridor, I receive a prompt to locate corridor baseline. What is a "corridor baseline"? In SS2 there is a separate menu called Modeler in the PowerCivil window, but now this menu is missing in SS3.
The corridor baseline that you are prompted to select is the horizontal alignment; select the alignment from your graphics. Modeler > Roadway Designer has been replaced with the OpenRoads Corridor Modeling command.
There is a video available which demonstrates a workflow for creating a corridor using the OpenRoads tools. You may find this video helpful as an introduction to OpenRoads:
Video: Simple Workflow for Corridor Creation