Reminds me of my 1st house; down cellar there were cross member beams about 25 ft long, crossing in the middle, with one tree-trunk size timber holding it up in the center. The ants had gotten to it and it was crumbling. I constructed two 16' x 6"x6" (3 2x6's) beams laid out on the floor with jacks ready to put up every 4', sent everyone out of the cellar and knocked over the center post with a sledgehammer, fully expecting the entire house to cave in on me (had a football helmet on, haha!). It did drop about 2" and held, so everyone scrambled back down and we hoisted up the beams and got jacks under them and got good support under the house (there was enough cross beam support - 14" hand hewn timbers, to have both beams pass each other in the middle). Eventually we raised the center of the house about 8" over a year's time to level out the center of the house much better, then remodeled the front room into a beauty parlor for my wife. Minimal wall cracks by turning the jacks only a ¼ to ½ turn every few days letting the house adjust to it slowly. Sold the house after about 8 years, moved to a different county, but will never forget knocking out that center post, holding my breath waiting for the house to fall!
In your situation what you could try doing is having a second ibeam cut to a wider stance than the present distance between support 2 and 3, remove one (say 2) support and put the second beam up, then post 3, giving greater distance between the two but with double the beam support. Consult an engineer or house builder 1st for recommendations, but I think it could be done.
