Yes that Huberman episode was very informational and broke the physiology of breathing down well! That's a good idea, give 3 seconds a try, and gradually you should be able to increase the time! 

BTCFC

health

Box Breathing

Really interesting to know that works for you. I just started with the cyclic sigh (deep inhale until out of are, one short extra inhale and deep long exhale). I'm also going to try the box breathing. According to the exercise by Huberman I could do 5 seconds, but I can't inhale 5 seconds, so I'll try 3 first.

Here is more information on the video of Huberman you also shared:
https://stacker.news/items/439822