if stickers don't work with him, then they don't work. it's got nothing to do with you doing anything wrong, he's only 3. your praise and a smile would, i'm sure, be enough. as you say he doesn't get it but don't you think it's because he's too young to understand? you are his universe at the moment, it's you and not stickers that matter to him.
hope you get to use the stickers as band aids for the house and get down on the floor with him and praise him from your heart.

Thank goodness some one else has the opinion that 3, 3 and a half and four is still just coming out of baby hood. Why do people (schools, pre-k, etc...) think and feel it is okay to make you feel your child is behind? The very idea that people think they know your child better than you do often makes me doubt myself. But no more! Anyway, that was my point. Kids don't need stickers, they need you. I hope so called experts understand that all kids will be different from each other and if they were not the world would be a sad and boring place indeed.

LOL don't get me started on the establishment and young children :)
i think children aren't allowed to be children for very long now adays, that's one of the reasons i home educate.
good luck love :)