The flat on your tool for a 3 pitch Acme thread is 0.1236 I think. This means you will have to wipe out 0.3333 - 0.1236 of thread root, or 0.2097. I try to take less than 1/2 of the tool flat for each pass (0.1236 / 2 = 0.0618). So, you will need to adjust your Z start point and end point negative Z 0.0618 until the entire 0.2097 is covered. Also it is recommended to have 3 times the pitch as lead in for threading, I will change that below.

G00 X4.2 Z0.9999 T0909
G76 P020000 Q0050 R.0005
G76 X4.4 Z-2.0 P0100 Q0472 F0.3333

G00 Z0.8763
G76 P020000 Q0050 R.0005
G76 X4.4 Z-0.0618 P0400 Q0472 F0.3333

G00 Z0.8145
G76 P020000 Q0050 R.0005
G76 X4.4 Z-0.1236 P0400 Q0472 F0.3333

G00 Z0.7527
G76 P020000 Q0050 R.0005
G76 X4.4 Z-0.1854 P0400 Q0472 F0.3333

G00 Z0.9381
G76 P020000 Q0050 R.0005
G76 X4.4 Z-0.2472 P0400 Q0472 F0.3333