Without pictures it is hard to be sure of a good recommendation. I'm guessing that the original cross members are 3" rolled channel, not structural channel.
How did the cross members get bent?
I've seen people replace their original trailer that needed a rebuild with a better looking "donor" trailer that will need a rebuild in a few years and think that they did great. I don't have the heart to tell them their original trailer was much better suited for the boat. Some boat trailers have the axle mounted on top of the leaf springs. That helps keep the trailer low for good handling and easier loading. Unfortunately, with the cross members bent downward, to fit the hull shape, there isn't much ground clearance. I've seen a lot of cross members bent that way.
If the original cross members are made out of rolled "C" channel, and that material is not available, they can be replaced with 3" structural channel. It is hard to give advice with no pictures and minimal info.
