The pale one is probably in shock now and can easily die.
It can be because it cannot find sleep in the tank.
The two others sound strong. I have had both good and bad experiences with LFS and if the look bright and fine now, no signs of starting to be worn, it sounds as if the otherwise healthy fish just did not handle the stress of being in the button of the hirachy.
I would say that you can avoid them if the others seem unhealthy as well, but, in nature they spawn huge numbers.
Some of the individuals will have genetic weaknesses and similar problems that means they would die naturally.
It is no use to look for the exact reason of why each fish die, perhaps this pale one is sick. It is dying if you ask me but it could just be the 1clownfish that was weaker than most.
I have read people say that you can leave them at the seller for a week and ask for a refund if they are getting less healthy.
I must say that I have done the opposite and bought the fishes faster to avoid them hanging out in at the LFS for a further week
1time I left two clowns at the LFS for a week. An anemone died in their tank and wasn't removed. The two fishes was not looking well. I tried to bring them home but they refused to eat and they were dead the next day.
I was later told that ammonia poisoning will destroy their organs and all such sad details.
Don't be too hard on the LFS because of what happens in their tanks. They are holding the livestock temporarily and they are dependant on us to come and adopt the pets we like.
My personal opinion would then be: find another if you were not actually happy with these and if you were 100% happy with them, don't leave them at the petstore - they are waiting for someone just like you to come and rehome them to somewhere much better than the waiting-tanks !