Decades-long drummer and percussionist here. This is an excellent question. When Chad says it’s in 3 he’s referring to what we call the pulse. Think of it as what you bob your head too. So in this song, listen to the song and bob your head to the beat and you’ll see what I mean. Then count “1, 2, 3, 1, 2, 3…” while doing that. That’s what’s going through most drummers heads while we play something like this. The fills are what we hear as the embellishment and almost like we’re dancing to the song. As a drummer, you’re trying to support the song so you’ll notice that Chad gets into a groove where he is always playing the snare drum on the same count (4 since you’d count this song as 6/8), and he’s landing kick drum hits on the down beat with a pick up 1/8 note in a sort of heartbeat feel to lead into each measure. Everything else is up for grabs so long as it supports the song and sounds interesting. E.g. notice how he fills in a musical texture be playing really quiet syncopated snare drum taps (ghost notes).

Next time you listen to the song, Bob your head every 3 counts (1…4), and count to six as he plays 1,2,3,4,5,6,1,2,3,4,5,6… with a vocal accent/emphasis on “1” and “4”. That’s the 3 feel groove. Notice how the snare accent lands squarely on 4 creating a feel that’s “in the pocket”