Wow. I just looked up the data and Hodgdon.com and you're right, they say 3.4 grains is a starting load. I have no idea why they would start so low (12600 CUP) because that's not even a respectable .38 Special load. Then Lee just copied Hodgdon's data.
I have a 1997 Winchester load data book. It says a max load of 231 in .357 magnum with a 158 grain lead bullet is 6.7 grains (42500 CUP) Don't take my word for it though, try to find another reference.
I use Herco powder in .357 Magnum with cast bullets.
"The only way to stop a bad guy with a gun is with a good guy with a gun"