I was an idiot and didn't fully prime my torque converter before installing it. I put in about three quarts before putting it in. Now that everything's put back together, I don't have any gears, everything but park is neutral. I'm assuming lack of fluid in the TC would cause this issue. How do I get more fluid into the TC without removing my transmission again?

Funnel into the transmission dipstick tube.

Bad policy in general.

Run the engine a few seconds. That's all it takes even if the converter is put in dry. The pump constantly delivers fluid into it. If this is not happening, it's because the pump isn't working.

The transmission as a whole takes about 11 quarts. About 9 should be enough for it to at least start doing SOMETHING. If you've put that much in it, and nothing is happening, then you have a pump problem.

Yeah ive put my converter in dry in my th400 and it filled up fine upon starting. If the trans has fluid in the pan to the appropriate level it should have gears

convertor dry install is fine.
on a fresh build, no fluid, i add 4 qts to pan, start enginerun for 4 seconds turn off. add 4 more qts, restart engine, check for leaks, then using dipstick fill to correct on stick.

never done it any other way, and have probbaly done 150 trans jobs if i had to count

When I bought my stage 2 700r4 I was told by the vendor it was okay to just leave the converter dry and that it didn't need priming, fill the trans with 4 quarts with the engine off then add 7 while running and you'll be right where you need to be. If you did a filter you might have left the old o-ring in the trans and when you put the new one in it doubled up and snapped the plastic tube when the pan was bolted down.