FRP coefficient of thermal expansion is about half that of steel (0.0000044"/"/F vs. 0.0000084"/"/F). Thus assuming you install the 'glass @ 70F and the panel heats to 140F in service (hot summer sun) and the longest joint in your setup is 36", then differential expansion will be about 0.010". 1/100th inch is plenty to overstress the bond between polyester resin and steel giving you a nice, long crack, regardless of the mechanical aids you incorporate.
Your design is pretty involved and will require some skill to pull off even in fiberglass. I suggest you take a welding course @ your local junior college and develop the skill to MIG weld sheet metal. It is a really easy skill to learn. Then you can do the job right and you will have a skill that will last a life time and that is one of the great side benefits of do-it-yourself hot rodding.
I would do that job using exhaust tubing. Someone out there surely does bends in 3" tubing. A good source to investigate is exhaust system suppliers for big diesel trucks.