Of the three (3700X, 2700X & 3900X) 3700X runs the coolest with it being about the same as 3900X up to 60w (3700X is 65w TDP; 2700X & 3900X are 105w TDP).
But like I said before, we need good case airflow so cooler(s) receive air at or near room temp and not pre-heated by components' heated exhaust air. Because every degree warmer the air into cooler is translates into same number of degrees hotter component will be. So if room is 23c and air into coolers with good case airflow is 23-26c and component runs 65c @ 100% load everything is cool and quiet, but if case does not have good airflow and air inside of case is heatig up (because case intake airflow is mixing with component heated air), the airflow into coolers will be something like 30-35c and component will be 72-76c. This is assuming case air into components is only 7-12c above room. If it is 17-2c2 above room components will be 82-86c.
A few years ago I helped a mate build a gaming system in SilverStone Milo Series ML08B 380x87x370mm (WxHxD) (12 liters). It took some trial & error testing and a couple of fan changes to get it all cool and quiet, but ended up gaming without headphones and just barely hearing system setting a couple feet away on desktop.