Yes, you can switch any age of dog. Raw is usually much easier on the digestive system than unnatural dehydrated foods, so it is usually a case of the older or the sicker a dog is, the more apparent the difference real food makes.
At 3, your guy is far from old, of course. But that doesn't lessen the likelihood that he'll do well on raw or adapt quickly to it
As for treats, you can use anything you like. Many dogs enjoy bits of raw fruit as treats. For those that don't, it doesn't stop you making your own stuff - for example, cubes of cooked chicken (just grill some chicken, then dice it up), or liver treats - which is basically just cooked bits of liver cut up. You can do the same with any meat, irrespective of what diet you feed. I don't personally recommend raw meat as treats - very messy and hard to keep in your pocket