A lot of questions and all depending on budget of course. But on the car rental, if you are spending a day or so in SJ, then I would push off getting the car until you are ready to leave, otherwise just leave it in hotel parking (depending on what hotel).
Also, in SJ, I like the Taormina for picking up live. Sportsman did not have the selection that I was looking for anyway. A little more money, but easier to find what you like.
In Jaco, the Cocal is always good. You will find everything you need at the Frog bar or the beach bar. Just have fun and ensure that your hotel will take unregistered guests. Try Hotel Jaco or the Cocal for hotels that are easy, close and open. There are a few condos there from the Palms on the far end to the Diamonte del sol and Vista Mar right there at the Cocal.
On the apps, the best one that I have had work for me is using Tender. I have picked up some on Instagram and one or two on Facebook. But Tender is a solid hit. Just look for those open to short term.
Threesomes are typically easy to get into. I picked up two girls at Oz Sports Bar (near Cocal) that were eating there and they were open to it. I only really wanted the one, but felt bad leaving the other so me being the helpful person --- I brought them both home.
In the end, it should not be a problem at all.