You can replace a 2 x 3 with a 3 x 3, IF the original tube was laying on it's side with the 3" being horizontal. If it was, the housing where the tongue is attached will be 3" wide inside. If it isn't, then You'd need to stay with a 2 x 3. Normally, the biggest size on a rectangular tube will be verticle, since that's the strongest direction. Square tube is more commonly used now because it's cheaper from suppliers, due to larger quantity of production.
You don't tell where You're located, so in general, look under metal suppliers in the Yellow pages. For a smaller trailer, like most tilt jobs, a 3 x 2 tube with a 14ga (.083) wall would be sufficient, but if You want extra strentgh go with an 1/8" (.125) wall thickness. Co.s such as Metalsupermarkets, Metals online, Alro, should have what You need. The one disadvantage of a rectangular tube is the supply available, in comparison to a sq. tube. It also makes the likelyhood of finding surplus considerably lower. Also these tubes normally come in 24' lengths, and most places will charge $25-40 to cut to size, since they don't want to have "drops" left. Metalsupermarkets sells by what ever size You need, with NO cutting charge, so if there's one in Your area, that'll probably be the best choice.
Good Luck and hope to see the conversion on the site.