I would recommend doing one at a time. I've been doing this for 1 year, so take this recommendation with that in mind.
For more detail, I just added a hippo, sailfin, purple, and whitetail tang to my tank (very similar to yours). I had a 40 gallon quarantine tank. The purple and hippo were in the lfs for a few months, they looked great, so I added those directly into display. After a month with the sailfin and whitetail in quarantine, the hippo was showing ich... so I removed the sailfin and whitetail, and moved the hippo to quarantine. I would not add more than 2 tangs to a 40 gallon tank based on how much room they needed.
The whole process was stressful, and eventually, I didn't add the tangs at the same time anyways, which is what I was attempting to do. I would say if you have a quarantine tank for each fish - in your case 3 - that is the ideal approach to add them at the same time.
However, being that you have 1 tank, adding them 1 at a time is going to be better for your fish and the display rather than trying to add them all at once.