I wouldn't take my boys to see it (they are 5 and 3).
..but I haven't let them watch the Terminator movies either and I know some other 5 yr olds have seen that. I'm not a good judge of ages but definitely (IMO) not a 3 year old. As for if the movie was good or not, I had a good time. I am not a movie critic and I haven't really been to the movies in ages. I liked the 3D effects on the IMAX theater..I didn't get bored or check my phone or watch.
I hope that you all enjoy the movie and definitely let me know what you think!