Reducing a 3/4" Male Flare to a 1/2" Male Flare
You've got the 3/4" flare x 1/2" NPT hex nipple, round backwards. The flare end should go in the swivel nut on the far right of the first photo, while the National Pipe Thread (NPT) end should go in the coupler between the two flare hex nipples. 

From left to right, it should go 1/2" flare x 1/2" NPT hex nipple -> 1/2" NPT coupler -> 1/2" NPT x 3/4" flare hex nipple -> 3/4" flare swivel nut.

To do this in one fitting, you'd need a 1/2" flare x 3/4" flare hex nipple. But I don't think they make those.

Just make sure you use pipe dope on the NPT threads, you tighten all the connection enough but not too much, and you pressure test all the connections.