OK, First off, What vehicle are you working on?
Second, when it WAS WORKING, what did it do when you stomped the throttle?
Did it shift down into 'Passing' gear?, From 3rd to 2nd, or is this a new install that isn't working correctly?
Third, Is this a fresh rebuild or tired, used transmission?
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No throttle valve on a TH 400, electric kick down solenoid.
If it's not hooked up or working, you labor the hell out of the direct drive clutch with low line pressure (Low engine rpm, lots of load, too tall of gear, recipe for burning the direct clutch up),
The two other things that can effect 3rd gear are, in order of importance,
Governor. half round 'Dome' cover over the governor, the governor will pull out of the transmission, kind of 'Un-Screws' as the drive gear comes off the transmission shaft teeth.
If that governor's plastic teeth are damaged, or if something is keeping the governor valve from passing fluid pressure (Like crap caught in the valve) you won't get an Up shift.
The next place I would look is the Vacuum Modulator Canister and it's spool valve just inside the transmission.
Some times that vacuum modulator will quit working and cuase odd issues,
Other times the spool valve the vacuum modulator works (just inside the case) will jam up and cause problems.
Vacuum pump or sucking on the diaphragm with a clean piece of tubing will tell you if the diaphram is ruptured and leaking,
Vacuum pump will tell you if it's working correctly,
And taking the spool valve out will tell you if it's jammed,
(Obviously, if it's jammed in the bore, THERE'S YOUR PROBLEM!)
They slide out pretty easy, so have a look to see if there are signs of burned clutch material stuck to it...
This REALLY isn't rocket science...
Like I said, there are DOZENS of those $20 rebuild manuals out there with trouble shooting sections, pictures, ect.
Picking one up would be a good idea,
Along with getting it to one of those $20 or Free inspection transmission shops where they have all the tools to do the proper testing and find EXACTLY what the issue is.