How much weight can this Freightliner Columbia 3 axle handle.

90k is nothing really. Not gonna hurt it.

 

If you drive it easy without dumping the clutch on hills where you must stop on them it can haul closer to 100,000 every day with permits and keep on doing it for a long time?

 

Load the wagon,,,just make sure you can stop the dang thing.

 

My 96 IHC was spec'd the same and that was for hauling up to 9 axle (158kgross). I thought nothing of running 120k gross on 6 axles.
If you have 40-145 drives then your gears are good for 145 gross and us in Canada use that set to pull super-B trailers up to 140k.
With your light Columbia and a 40 ton with a pin-on 3rd axle you should be able to permit up to 114k almost anywhere in the lower 48.
14k on your steers
40k on your drives
60K on the trailer
That should let you put about 75k on the deck if you can axle scale it.

 

That truck can handle more than you will ever want to put behind it.

 

6 axle with a Iron wolf brush hog 106k net. Piece of cake.