Ok so I got tired of trying various combinations and I just read the source to see what's what.
There are several factors on top of the obvious. Unlike normal stat points, the husband events can go out of order and fuck your playthrough.
But first, for those who don't care about details:
Have fun.
Some details now:
The key point for the threesome is to hit the exhibition end (10 points, route completed), never threaten the missus, AND for $husbandplayed to be at most 1 less than $husband. That latter variable changes everytime you hit a "Wifey learnt a lot, do you want to please Hubby?" choice, whether you say yes or no. The problem is, it changes to different values depending which event gave you that choice.
- husband=1 after 3 events.
- husband=2 after 6 events.
Then it all goes out the window:
- husband=3 after maxing Wife's vagina route (5 points) ONLY if you tell her to take the pill; takes 9 events at the earliest.
- husband=4 after maxing Wife's BJ route (5 points) while also having 2 worship points; 8 events at the earliest.
- husband=5 after maxing Wife's exhibition route (10 points), which cannot be done before the next one; 23 events at the earliest.
- husband=6 after getting 2 points in bondage (required to finish the exhibition route...). You must let her fall during the 2nd bondage event for it to happen; 15 events at the earliest.
- husband=7 after getting 1 point in anal; 6 events at the earliest.
The last event you go through overwrites the value.
$husbandplayed is easier to manage. You can only get it up to 4 right now, by choosing 'yes' and letting the Wife please her husband for the first 4 events of the previous list. The others don't do anything about it. Maybe that'll change in future releases.
So, technically, you can do stuff in an order that lets you see everything, but with $husband equal to
3, 4 or 5 at the end. That's what you want, to be able to reach the threesome, because going anal last will lock you out.
Since you must hit the exhib route end, 23 events are required, so you will at best be able to reduce $husband back down to
3 4 afterwards by leaving the
vagina blowjob route unfinished until the last moment. So you will have to let Wifey please Hubby at least
twice thrice to hit the threesome route, if you care about that.
Edit: Rechecking each route's requirements, the first bondage event forces you to finish the vagina route beforehand. So you can't even keep that event for last; at best $husband=4, so that's a minimum of 3 for $husbandplayed.