Back off of the punishments - the consequences you are using don't fit the "crime".
When he wets his pants, have him change his own clothes before he can play more. If he cannot change his own clothes, gently let him know that as soon as you have time you will help him - but he cannot sit on anything but plastic chairs (if you have them) until you are ready.
Stopping play rather than taking things away will work better and is a direct consequence. It is a natural time-out that he is causing himself.
These consequential - non-judgmental or angry - time outs will be natural consequences of the fact that he is not paying attention to his bladder. Don't call them time outs, just get into the habit of making him wait for your help OR having him change his own clothes before he can play. If he cries, instead of telling him that it's his own fault, tell him how sad you know he must be to have to wait to play. He will appreciate your sympathy which will confirm that you are not mad at him.
Eventually, he will get tired of waiting to get back to playing and will get to the potty. He will also get that you care about his feelings and won't need to cry to get your sympathy.
One more thing: Your attention for this is what he is really seeking (I know it's weird, but kids seek attention at this age even if it is negative). Don't give it negative attention and allow him the ability to think through it himself. A three year old is smart enough to know that the consequences of their actions are fixable. Especially something like this.
Also, because he is not your son, he will be doing lots of things to test you to see if you love him as much as he wants to love you. Your best shot is to validate his feelings all day long - every time he cries, every time he laughs, every time he whines... Your job here won't be easy if you resist his calls for your love. Love him as hard as you can, even if it feels impossible because he is not yours. He needs to know that you love him as much as you love your little baby.
Read a book about Blended Families as soon as you can P.. You may feel like you are in over your head for a while.