3 sides is fine. The important thing is to attach to the spars. Attaching to EITHER rib is a bonus. The web is there to reinforce the spars and turn the structure into a type of I-beam. Well OK, a C-beam in this case. The extra web greatly stiffens/strengthens the assembly and reduces flex.

I know you are using TiteBond and I heartily approve of that, great glue.
When I glue the doublers I will apply the wood glue as needed and then put just a drop of CA at each end to maintain alignment. Once the CA sets, put wax paper above/below the piece and weight it down evenly until dry. NO PINS! Put the CA in a place where you won't be sanding later.