I just uploaded your macro to see what you have so far. Is it not important that Arcane shot fires after those two, like is there a proc, if so you could think about a cast sequence. If it isnt important you could just add arcane shot under the other two and maybe set it as a priority cast? or if those two spells need to be fire on cd maybe consider moving them to the pre macro and leave arcane shot in the normal macro or so? or possibly more arcane shot to post macro? Im not sure if there is exactly a way to shoot only those two but if they are on cd you shoot arcane shot without doing one of the above. you could also try to cast sequence concussive shot, arcane shot and barbed shot, arcane shot but having two separate would fire concussive, arcane, then barbed and arcane most likely so its also not an answer really. I will post a macro below to possibly help, I have barbed shot first but i think you should maybe switch the order so concussive is first so they are slowed immediately?
Sorry i couldnt help you really but hopefully one of these guesses could work.
Here is a quick macro i threw together to maybe elevate the problem, or at least its a start.
Click here for code