Voadam
Legend
The closest I can find in the DMG on three rounds is the discussion on page 278 about averaging damage ouptuts over three rounds for judging the damage output of a monster with spiking variable attacks when calculating CR.Interesting. Is that in the DMG? Got a page number?
If a monster's damage output varies from round to round, calculate its damage output each round for the first three rounds of combat, and take the average. For example, a young white dragon has a multiattack routine (one bite attack and two claw attacks) that deals an average of 37 damage each round, as well as a breath weapon that deals 45 damage, or 90 if it hits two targets (and it probably will). In the first three rounds of combat, the dragon will probably get to use its breath weapon once and its multiattack routine twice, so its average damage output for the first three rounds would be (90 + 37 + 37) + 3, or 54 damage (rounded down).
This of course does not say that CR assumes three round combats.