Three and a half IS old enough to know not to do this.
But, there is a reason she's doing it. (it's probably just experimental)
So, before showing that you are mad... (cuz who wouldn't be mad) I would try to find out why some kids do this. I know really little kids do it, because they could reach it in their diaper, or they took their diaper off and then pooped.. Then played in it. But, typically a pre-school age child doesn't do this.
Anyway, my point is. There is a reason for this, and I can't even begin to guess what it is, but she doesn't know either. So, don't even ask her why.
Serioulsly, can you ever remember a time when an adult asked you "Why did you do this?" and you answered with anything other than "I don't know". "I don't know" is the universal answer to "why did you....".
*funny story to make you feel better*
When my niece was four, she pooped into a ziplock baggy, then she and her twin sister kept it stored in their night stand for several days before their mom finally tracked down "the smell". She wanted to send them both to counseling and was sure they were going to turn into sociopaths.
And. They are kinda weird. But, a good weird. All Geeky and into science. One is in college to be a forensic scientist, and the other is in pre-med. Maybe poop investigation is important for those later years.