Generally an hour in the mash is enough time as long as 1/2 of the grain bill is 2-row. An all 2-row mash can convert in 20 minutes. You need to cook (as if you intended to eat them) rolled oats prior to adding them to a mash, flaked oats do not need to be cooked.
Mashing 3 hours shouldn't be necessary, but won't hurt. Some people mash over-night just to break up the brewing session into two shorter periods.
Once you boil the wort the enzymes are history.
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