I have an Arlo security system but it’s not Pro. Will it synch with Smart Things 3 hub?
I’m not sure what you mean by “synch” in this context.
But there is an official SmartThings/Arlo integration for several different Arlo models, not just a pro.
See the official “works with smartthings“ page.
And here is the official supportbase article which lists the model numbers which work with this integration:
Synch was just a bad choice of word. I guess pair or network may be better.