While adding additional vehicles to the production queue, you can only add one at a time. What I mean is if you open the window to change the line, and add a second vehicle type, you can only add one, when you select the third, the option to add stays greyed out and you can not add it.
In this picture, I opened it, Added in the Compact, then went to add the SUV immediately after, and you see the option remained greyed out, and I could not add it. If I closed the window, and select change again, I can add the third one in then. Small glitch, and not game breaking.