The rules:

So if the team with the 8-seed in the conference is ahead of the team in 9th place by more than 4.0 games in the standings, no play-in tournament is needed. That will not be the case with the Grizzlies sitting 1.5 games ahead of the Blazers with three games remaining. If the 8-seed is 4.0 games or fewer ahead of the 9th-place team in the conference, a play-in tournament will decide who gets into the playoffs. It will be a best-of-two series, with the 8-seed needing to win one game in order to advance.
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So it will very likely be Memphis vs Portland vs San Antonio. That is a very interesting way of doing it. It's impossible for any team to have more than a 4 game lead right now so it's definitely going to happen either way.