If you're here after the main TV series was released, then you're probably late to this, as I was, but it was better for me to watch this after I've watched the TV series anyway. At the very core, these three episodes (1 min each) ONAs are promotional material for the show that existed for as back as late 2018, so it doesn't serve any narrative motive however, be wary, it does spoil some tidbits so you're probably better off watching the show first. Otherwise, some reveals won't strike as hard. I guess they had no better character to feature in full color but Gojo, with servesas a clever decision but also comes with its own spoilers as well, so preceded with caution.