In terms of using sportspress pro’s Tournament feature – Off the shelf, the tournament feature does not specifically allow series between teams but I believe you can achieve this by using round robins and attaching those to the tournament – only for the 1st round*. You would then simply put those 2 teams (2nd and 3rd) that will be playing a best of series in that round robin attached to a specific bracket of the tournament and then hide other brackets in the tournament.
My Monday league is a 6 team league. So an 8 team bracket is out of the question. What I do is 1st and 2nd place teams after regular season get byes into the semis. Playoff quarter finals are 3rd vs 6th and 4th vs 5th. I hide 1 bracket on each side. This leaves just 2 match-ups in the quarter-finals. Winning team advances to the semis and we can choose a team to add into that other semi final spot (1st or 2nd place in season that got a bye) and then you have the rest of your bracket play out.
In your case, for a 3 team playoff, you would choose a 3-4 team bracket and hide one of the 1st round brackets so that it turns it into a straight semi-final and then the winner moves on to the finals. Again, the issue here is that for your finals, you don’t have the option to have a series (best of X games) between those 2 teams.
A work-around to all of this is to simply forego the tournament feature and instead create a new “Season” and make that your playoffs. Create the games you need for all involved. You can create the best of 5 matchup for 2nd vs 3rd and then your series for the finals and use the “Day” field to mark which round/game# it is. For my league. i added an extra “Event Spec” called “Game #”. My “day” field has become the “Week #” and then I also have the “game #” to number each individual game throughout the season and playoffs.
Not sure if this helps a little bit… if you have further questions about this, feel free to ask. Someone else might have a better option as well but this has so far worked for me.