I have front porch where three roof trusses with three cross slopes form the walls. How do I build the roof that covers the opening between walls, where each wall supports a roof sloping in different directions.
Being able to put a roof structure in this location would be fairly difficult due to the three sides facing this area and the water run off would collect in this area. Unless you wanted to plan on redoing the entire roof structure, I would look at alternative items to fill this space.
Depending on where you live and the weather you experience you have to plan accordingly. If you have a higher rainfall, then you need to make sure this structure won't impede the roof's water drainage. If you have snow pack, then you need something that is structurally strong. If you experience high winds, you need to make sure the structure is wind resistant.
Again, depending on your local weather, what I would suggest looking into would shade sails, or Fabric Porch coverings.
These structures are simple to construct, and fairly in-expensive. I was looking at this website for ideas, and I feel like this one is closest to the area that you are wanting to fill: