1. I think it's normal, if really frustrating
, behavior.
2. Some of that I wouldn't even consider "destructive," per se--like spreading flour all over the kitchen. That's just plain fun.
Really, though, he may be doing that for attention, or he may be doing it out of curiosity. If my 3-year-old did something like that (and she has) I would think, in her case, she was attention-seeking. I would think this because (a) she hasn't engaged in a lot of that sort of behavior as a 1-and-2-year-old and (b) she began occasionally doing things like that shortly after her baby sister was born in August.
3. Which I notice is also a factor for your little guy, right--you have a new baby? My 3rd child was born when my second was a few months shy of 3, and despite our concerted efforts to give her LOTS of attention, she has reacted to her "displacement" by occasionally acting out like you're describing--for the first week or two after the baby was born, she was peeing in her pants a lot (she'd been potty-trained for over a year at that point), then she continued making messes like you described, coloring on walls, coloring on herself...she once stripped naked and covered herself head-to-toe with Elmer's glue. I think it's just her way of dealing with her feelings about no longer being "the baby" and having to share us with another sibling. The baby is 4 months old now, and things have steadily improved. She's still a bit...different...from her pre-big-sister self, but it's manageable and generally doesn't concern me anymore.
4. Plus, he's three. And I think three-year-olds are a tad crazy anyway.
Good luck--I know this can be really frustrating, but I wouldn't be particularly concerned if I were you. Give him lots of extra "preemptive" attention whenever you're able and don't make too big a deal out of stuff the rest of the time.
ETA: As far as him destroying his sister's decorations/et cetera, I would say something to him about it, reminding him that his sister worked hard on it and that it was hurtful to her that he did that, but I would give most of the attention to his sister, telling her how sorry you are that that happened, maybe helping her make new decorations, whatever. In general, I would take his behavior as a sign that he needs more attention, and I would try to give him LOTS more attention--but not make it appear as a direct result of the behavior, if that makes sense, to avoid giving him the idea that ruining his sister's projects or making messes in the kitchen is an effective and appropriate way to get the attention he desires. Make sense?
