Our first trip to DL, our kids were 4.5 years and 19 months. They both loved their autograph books. My 19 month old ds was determined to see Buzz Lightyear. When we ran into him in Tomorrowland, he wasn't doing autographs yet. But my ds kept pulling the autograph out of the bottom of the stroller and running up to him, saying, "Buh! Buh!" He couldn't say hardly any words, but he sure got Buzz down pat. When I started to leave without Buzz's autograph, ds started to cry. So we waited. I wound up chasing Buzz all over Tomorrowland, when he came back from a break. But darn it, we got that autograph for my 19 month old.
So yes, I would suggest an autograph book for a 3 year old. If you want, you can get one that has picture slots. It doesn't have as many pages, but you can put a picture next to each autograph, of your child with the character who signed. I think it's just $12 for that style.