340 engine isn’t getting enough gas, why?

If she was going lean in third gear, there would be a lot of bucking and farting and misfiring/backfiring going on. You didn't mention any of that.

In the absence of drama,Verify the choke is coming ALL THE WAY off:If the choke mech is not coming off, the secondaries will not open.
After that; verify your timing systems.
If that checks out, time for a compression test.