Missing 3rd on 904 trans

If you tied the throttle valve back it will take a lotta lotta rpm to command a 2-3 shift. Depending on how far it is tied back, it could take 5000, and with 3.23s that could be over 85 mph.
However, if the governor is working, then the 1-2 shift would take about the same rpm, but now at 50mph.
If the governor was not working, then you would not get an automatic 1-2 shift either.
Since reverse is working, then BOTH the L/R band AND the Hi-Drum are working. In reverse the oil pressure is unregulated and if you cannot make reverse slip, then there is nothing wrong with the hard parts.
There are no bands involved in Third gear.
If, when the trans is scheduled to go into Third by the governor, and it goes into a false neutral instead, then the hi-drum is not getting oil-pressure. Remember, in reverse the pressure to the hi-drum is whatever the pomp is regulated to, or about 260 psi. But in Forward, it is limited to a maximum of ~*90psi depending on where the TV is depressed to. If the governor pressure is not more than line pressure plus TV pressure, then no shift can occur.
If you have locked out the governor on purpose, in any method not detailed with the shiftkit, then it is possible for the governor pressure to leak away, and upset your line pressure. Since the Throttle Pressure is a function of Line Pressure, then it too is compromised.
If the wrong spring was installed in the 2-3 shift valve, that could prevent a 2-3 shift
If the #3 ball is missing from the VB, the circuit will hemoragh oil pressure. Same if the accumulator busted.
If a too strong spring is installed into the TV circuit, the pressure will be too high for the governor to overcome and the shift will not occur.
There are seven small balls in the VB ; The main relief is .375 . Of the remaining six, Five are (5/16), while one is a hair bigger, 11/32 (IIRC) bigger than the rest, and it goes in the bigger cavity, close to the relief valve..
if the VB is off, now is the time to do an Air-pressure test, to prove the hard parts are working..