Don't know if you're talking 3 in-game days or 3 RL days, but I'll try to answer keeping that in mind.
I've found it pretty reliable that your date will invite you back to their place after a dinner date. How much they're willing to do depends on how well they know you unless you turn off dating sim progression, but both of you should get off by the end.
The only way to get threesomes pretty regularly is to become polyamorous, or at least have an open relationship, and invite two or more of your partners to "hang out" somewhere as part of a date. They should all degrade into a sex pile after things get going.