Bridge looks great, if you wanted to add strength easily to it at this point in time you could bolt on some filch plates (fish plate) or some 3\4 pressure treated plywood, some of the strongest field fab beams incorporate plywood,2x12s bolts and construction adhesive as the ingredients. Right now you could sandwich the beams with the plywood use lots of exterior grade construction adhesive and carriage bolts. Typically its better to sandwich the ply in between the 2xs but I think it would be okay to do it on the outside. Or if you want you could bolt on filch plates that over lap the lowest point of deflection on your bridge. If you do consider the ply option not that you need to , just do it on the inside of the outer beams and both sides of the inner beam. Is it to late to consider stripping the tops of your beams with Ice and Water Shield? If its not to late this is a good way to go so that when you nail the decking down it will seal around the nails stopping water and ice from penetrating the top of your beam via the nails causing premature weakening and weater damge to your beams . Having said all that your bridge looks real good and I am jealous that I have nothing to build a bridge over.