I'm talkin by memory, so it may not be accurated.
The threesome Sarah offers is the one the MC has to choose the third companion from a list, right?
Well, I always choose the same one, knowing her sluttiness/ obedience are in slave levels.
The surprise during meeting can be either having sex with her at the office, show you her new breast size, or inviting you to the night club to get a private dance from one of the strippers.
When I decide to play this game (skipping 2 or 3 releases) I just, you know, play. Focusing about triggering an specefic event can be very frustrating and a huge turn off.
To ease things a little, once reached an advanced stage in the game, you will surely have created a serum similar to this:
This kind of long-lasting serums are most useful for sluttify the whole city quickly, via "The Downtown Distillery - Offer a drink for...", to get rid of the City Administrator threat once she accepts taking a coffee (so when you "Try to seduce her", she will eventually leave without fining, or taking your serums/ designs away.)
Is better to have a batch of those with specific formulas for different purposses, and with little effort the game will become even boring.