To stop your 3 year old boy from taking his diaper off you can take some packing tape or duct tape and wrap the tape around the diaper a couple of times.
The tape is hard to remove even for adults without use of scissors so the toddler or 3 year old won't be able to take the diaper off.
I've had to use packing tape around my 3 year olds diaper to keep him from taking his diaper off and it helps keep the diaper held up better as well.
Also keep the toddler in pants with a belt to keep the toddler from getting access to his diaper.
The harder you make it for the 3 year old to get access to his diaper the less chance he will have at taking the diaper off.
Also see if the 3 year old is ready to use the toilet.
If the 3 year old is taking his diaper off he may be trying to let you know that he's ready to use the toilet and stop wearing diapers.
So it would be good to try and get him to use the toilet.
But if not then use the tape method or use some footed pajamas or other one piece pajamas that zip up the back to keep him from getting to his diaper.