904 no 3rd gear?Why?

Well 3rd gear is actuated by the front and rear clutch packs being applied simultaneously. If the front clutch pack is not working you will not only have 3rd gear missing but reverse won't work either because it also engages along with the rear band to apply reverse. If reverse works you can rule out the front clutch. If the rear clutch isn't working you won't have any forward motion so you can rule that out. The only thing I can think of is I remember seeing in the instructions of a Trans-go TF-2 kit that in the valve body there are several check balls and it says you have to make sure and use an 11/32" ball for the large ball or you won't have 3rd gear. A 3/8" ball will fit where it is but won't let 3rd gear engage. You might want to check that.