It's funny how different two siblings can be, isn't it. Sounds like your first was/is an "Angel" baby! I had the opposite my first (DS) was a nap refuser and fighter. I was ready for the same with my 2nd (DD) and it was such a surprise when she started self-settling at 8wks old!
My DS needed movement in buggy,car or rocker chair to nap until I sleep trained him at 6mo. The things that helped us were:
1) a consistent nap wind down routine- before every nap I would do quieter activities with him, then go into the bedroom, blackout blinds and curtains & low lighting, read a story, play the same short bit of music then settle him into his cot in a lightweight sleeping bag and turn lights off. At first it felt like it was just for my sake but repetition of the same wind down really helped in the end.
2) I did a combination of an adapted sh-pat & gradual withdrawal. So after WD routine I would cuddle him until calm then put him in the cot and continue to hold him there and sh until he fell asleep (patting and picking up were too stimulating for him and kept him awake longer).
Once I knew he could sleep in the cot I would cuddle him for shorter and shorter time until I just went straight from WD to into the cot & holding there.
Then I started letting go of him to let him settle himself earlier and earlier so holding until sleepy but not asleep, then after that worked just have a hand on mattress next to him, then just standing by cot, behind cot, by the door, outside door etc.
It took a while but it was worth the effort as he self-settled to sleep ever since, even when we're away from home. It does need time not being interrupted though- Have you got any days when your 3yo is in childcare? Good luck and please come back if you want to chat or just need some support amd encouragement.