venture transmission problem

I have a 2000 venture. 175,000 km,s. I've never changed tranny fluid.
Since a week ago, it won't change into 3rd gear. Will fluid and filter replacement correct this?

Remote possibly, but not likely. It really depends on what is not working. If it's a valve that is sticking, it's a possiblity. But if it's a worn valve that's leaking or band or clutch plate that's shot, no amount of fluid, filter, or addatives will bring it back.

You have never changed the fluid or the filter, but have you checked the transmission fluid level? I don't know if changing the fluid/filter will correct your problem, but I once had a transmission that didn't want to shift gears (AMC Pacer) and just replacing the transmission fluid fixed it.

If it isn't throwing out any codes I would give it a try and change the fluid/filter and see what happens.