Really similar story here...just a few short weeks ago my third (then 3m old) was really struggling with his naps, and with two big sisters around it didn't help! He is now much, much better, with the help of a very dark room, a full swaddle, white noise and a consistent (but quick) nap time routine to get him into his cot. While we still have issues with waking up at 45 mins on some days, he will often resettle if I catch him before he wakes up too much. I've hardly used the sssh pat since I helped him to sort out his naps, and now I can genuinely say to the girls that I will be 5 mins and they know I mean it.
If you can find a combination that works for you, just stick with it! Good luck...