Hi @JustAddCoffee91 and welcome.
@PinkDaydreams, I have the Galt Dinosaur reusable sticker book which is similar to the Melissa and Doug book, but I think the M&D one is better, but it's fine if you want a change. You can get quite a lot of 'puffy' stickers in bulk on Amazon and they are OK too although can get very small.
If anyone likes the Gruffalo you can get a reusable sticker scene book on Amazon. I also got a Baker Ross nursery rhyme sticker scene one too.
Totally agree about outdoor time for 3 year old. We take DS1 out for a walk after tea now the weather is a bit brighter. It helps to calm him down.
Now, call me bonkers but how about meditation/mindfulness for 3 year old? I love all that and I had a thought today I would try a little with DS1 before bed, breathe in ahhh and out ahhh, you are nice and relaxed, now its time for sleep, you will have a good sleep etc etc. Hmm, I do sound mad now, going to keep trying it to see if it calms him. Don't think it would work on younger DS2 though.