How long do you let a turkey rest after cooking?
The purpose- as with any cooked meat- is to let the meat firm up so it doesn't release juices when you cut into it.

In the case of a turkey it also helps to let it cool enough to not burn you when you are carving and eating it.

Both of these goals will be met in 30 minutes to an hour.

I don't know why that chef would recommend 3 hours. At that length of time the turkey would start to approach room temperature and would be less appealing to eat as well as start the clock on the danger zone.