Well, a 3 yr old would already understand it if you tell him/her that what he's/she's doing is wrong...it's always better to start teaching kids at an early age regarding proper behavior...I think it would be best if you involve your kid with gardening so that he/she can enjoy the activity with you. Good luck :)

Spanking.

In 10 years, stomping plants will the least of your concerns.

Does your three year old like butterflies or ladybugs? Boy or girl, most children like bugs...especially in cartoons and Disney movies. Explain that the bugs need the flowers for food...and sometimes a home. If you tear up the flowers, you're hurting the bugs.

If that doesn't work, I'd try reyesuela's suggestion.