joint solder problem
The proper way would be to take the joint apart, clean it up and start over. You don't know why the leak is occuring. It may not be just that you need more solder in that spot. There may be a tiny grain of dirt in there, or maybe you didn't scuff up the surface enough, or applied too much heat such that the flux melted away in the spot. So without knowing exactly why the leak is happening, trying to reheat the joint and put in more solder will likely just end up making it worse or not doing anything. I'd say you should save yourself additional frustration, and just start over. If you don't have any flex in the pipes that lead up to the tee, you'll have to cut the pipe to take out the joint, and then get some repair couplings to put the pipe back together afterward.