The issue here is not your son, it’s the fact your husband and you are so far apart in parenting styles, that will be very confusion to a three year old. He will see daddy as th bad guy and mummy as the soft touch,
No three year old should be helping themselves in the kitchen that’s beyond ludicrous, and some of his behaviour, like telling your step son he’s not allowed in the garden or throwing things when he does not get his way is not good,
Children need boundaries, you and your husband need to sit down and talk and agree a compromise on the way to parent, because if you don’t your going to do some damage here.
Your job is to parent, to teach, to lead, and yes o a certain extent control at this age Ie what they can eat, drink, when they go to bed, what behaviour is acceptable.
If you and your husband are at loggerheads, it will simply damage your marriage and how your son grows up.