Hi , i am  not an expert on this , but  recently i done all the job at alignement shop and now is running good.
Does is possible that you have one broken shocks or spring on passenger ? 
Something that is not at right spot  after 3 times ? 
How may spacers you have between upper arm and frame ? 
Not a lot to work on ....but hope it helps 

Camber is off, like Al said. This can be caused by worn parts like upper control arm bushings and lower control arm bushings. Ball joints or other parts would have to be totally destroyed to affect camber.  Sounds like you ran into three shops that have no clue how to work on an old Mustang.

First, make sure all parts are in good condition.  This includes checking the shock towers for cracks or bent parts.  Then, look at the 3/4” hex nuts on the engine side of the shock towers. (Two per side).  Do they have additional threads sticking past the nuts, or are the nuts at the very end of the threads?  If there are no additional threads, the bolts have to replaced with longer ones (Moog makes longer splined bolts for this).  The extra length will allow a competent alignment shop to properly set the camber at zero degrees so the wheel is vertical. Any excuse offered by a shop is your signal to go somewhere else.

Installation of a Shelby export brace will help maintain alignment and should be installed prior to gettng it aligned. Only a Ford original or Scott Drake concours export brace will do the job. The Chinese versions that have a very flattened W shape are worthless. A real one has vertical ridges to make it strong.  Typically, if it is chrome, it is one of the worthless ones, although Drake has made chrome versions lately. They also make a cheaper version which should be avoided. Concours is the magic word.

Wag8142 wrote:

Thanks guys for the info .
Everything on the front end is new . Shocks, springs , control arms , tire rod ends , ball joints and 5 lug swap .I think I need to find a good front end shop. 

When you replaced the upper control arms, did you reinstall the shims from the original?
If not, that's your problem.  Current realignment shops no nothing about upper control arm shims.
 

Might want to look closely at your shock towers. Make sure there's no cracking around the upper control arm area.