Congratulations on your new DS
Yes I'd definitely try to put him down before he gets OT but this is the age where they do start to wake up more as you say, and it gets harder for them to drift off. Where's he sleeping? Some LOs do fine in the living area where you can keep an eye on them at this stage, others begin to need some dark and quiet/white noise so that they can shut out visual and sound distractions. SIDS recommendations are to be in the same room, but many of us find this impractical during the day especially when you have an older LO - how old is she? So you do what you feel comfortable with. Sorry, I digress!
Whilst some babies can drift off and self soothe from an early age, most do seem to need to be taught, which is what sshh pat does. It's a good idea to work on a consistent routine for naps and bedtime, which needn't be long, just something to show that it's sleep time, eg swaddling - do you swaddle? It can be really helpful for many LOs in the early weeks/months. If you're breastfeeding, 3 weeks would often be considered too early to use a dummy/doody, but if bottle feeding or if breastfeeding is going very well then it could be ok and many LOs find it really helpful (once breastfeeding is established, if applicable).
So, you're fine to just put him down and wait and see what happens. He may wriggle or fuss a bit and then drop right off. But if he's anything like ours, as he gets older he'll find he needs a bit more help from you. That's where shh pat comes in. Some people do it before putting them down, starting on your shoulder, and carry on until they drift off. I'd recommend doing as little as you need to start, but don't be discouraged if you find he needs more and more over the next few weeks - as he gets even older and has got used to you calming him with shh pat, you'll find you can transfer more of the work to him, so you'll gradually do less and less and finally he'll be sleeping independently
with a few bumps along the way...
Finally (for now) don't be afraid of prop naps when you have to, both to break OT cycles but also to get out with your older one, in fact my DD learnt independent sleep around the same time as DS (4-5mo), but only had one cot nap a day to begin with as I was out with DS. Do whatever you have to do to get through the first 3-4mo
self-soothing skills tend to appear around 3-4mo, although some do get there earlier. HTH, go easy on yourself
