Is it ok to pur a three year old autistic child in time out?

Yes, of course! I would recommend that you make it immediate and I mean immediately after the second the unacceptable behavior is occurs. Your child will need to see it as a negative consequence, too. If you see him or her enjoying your attention, the place you put him, etc., rethink time-out in its entirety. If it is working the way you want it to (he hates it), you will probably see him throw temper tantrums when he goes into time out. That's okay. He's expressing that it's undesirable. Stay with the rules that you set out with at first. Be prepared for the battle the first few times it happens. Behavior often gets worse before it gets better!