4x4 8feet long. I need at least 6 feet above ground.Quikrete 50 lb bags(love this stuff)
no mixing just add water when the post are leveled. But how many bags do I need for the three post.
in ground is 24" 2 feet.

I used this stuff to set a mailbox post a couple of weeks ago. I found that one 50 pound bag was about right for setting the post in a 2' deep hole. But the mailbox post is significanty smaller than your 4" x 4" post. I used a post hole digger to make the hole - if you do the same I would bet that you would only need about 3/4 bag per post. Which means for three posts you need 2-1/4 bags. So it seems like you have to buy 3 bags and you may have some left over.

Appreciate it, so much

A lot depends on the diameter of the hole. Buy 3 and take back the unused ones.