Good idea. We'll try this tonight in fact. He thinks its a riot! The other day the kid hid the pants I had JUST ironed to go to the pumpkin patch - he's an equal opportunity hider, and does it when no one is looking. The little, darling, sneak!
Welcome to parenting!
As other have pointed out, this is pretty normal behaviour, and is often linked to wanting tofeel some control----kids have so little that they get to control about their lives, so they find creative ways to get some. Your little stepson is probably feeling particularly vulnerable right now coping with a whole new person and parent figure in his life-----especally given how quickly your relationship with his dad moved.
He's being bounced from house to house (mom's and dad's) and has a new parenting figure in one, etc---that is a LOT for a 3 year old---so he seems to be handling it partly by exerting his own power where he can---forcing the adults to change their schedules (looking for things instead of leaving when they want to, paying attention to somehting about him instead of focusing on each other, etc for example), etc.
He's probably not aware of why he does these things, but if you understand where the motivation comes from, it can help you work with him.
Add in an attention seeking compenent, as noted by a PP, and you see where much of this come from.
I agree with those who suggest playing hiding game with him----pick a toy or item and ask him to hide it for you to find. Set up treasure hunts, etc----YOU initiate playing with him before he gets bored and initiates it (he must be left on his own fairly long to have time to get items and hide them that well with none of you having any idea where to look).
I also think it is really important to give him ways he can control his life, that are age appropriate and will help him cope (and build skills!). One great example would be getting dressed for the pumpkin patch. So long as the clothes are weather appropriate let him choose something--it's a pumpkin patch; he does not need to be dressed like minature ken doll---and if your mother in law complains, just tell her he choose his outift and is comfortable and let it go.
Or, you recently asked about where to buy his clothes---take him with zou and let him choose within the budget. Go up to a rack at Target and ask him which of the shirts he likes best,and get that one. (on the flip side, too much change all at once can also be really hard---I am curious why you don't just ask your husband and the children's mom where they have been buying clothes?).
Honestly, it is easy to get caught up in worrying about LOOKING like a good mom, especially if your mother in law is pressuring you---but if your concern is about are his clothes comfotable and weather appropriate and does he feel emotionally safe and loved rather than are his clothes wrinkle free and catalog cute enough and is he not slowing us down, you will be a much better parent.
Also----ask the kids' mom for adivce! (and dad---but I assume you've already done that). She knows her kids. She might have strategies that are working (and if not, if you two come up with some together and are consistent in both homes, yay, that is helpful to the kids too). Or ask Grandma---maybe if you quit trying to be perfect from day one and admit this is not easy and ask for her ideas, she'll be less critical and more helpul (and be sure you are not faslely assuming she is critical if she offers advice---maybe she is just experienced and trying to help both you and her grandkids)