I have a 3 year son who will be four in May. He is very outgoing and friendly, we never had any problems with him at all. Until, my husband and I decided to take a cruise.
I homeschool so he is with me and his brothers all day, but we are very active in our church and he has no problems going to "class" at church by himself. No crying, just runs off to class and does great. So we figured when we were going to leave for a week he would be fine. We found several homes for him to visit while we were gone. These were all children and adults that he knew very well, not strangers at all and he was very excited about it. He couldn't wait for us to leave.
Well, he cried every day while we were gone. He wouldn't eat, wouldn't sleep, cried at church, cried all the time. The only time he stopped was when he went to a friend's house where one of his brothers was staying also. When he had his brother with him he was okay. He stayed there the last three days and was able to eat and sleep.
My point in telling you this is to say that your son is simply not ready. If you don't absolutely have to leave him in preschool, then don't! He needs mom. Next year when he's 4, almost 5 he'll probably be ready. He'll be better equipped to deal with a day without you in the company of non-family members. But if you push it now you may have the opposite effect and cause him a lot of emotional trauma.
When we returned from our cruise my son was very clingy, which as I said isn't like him at all. He was like this for about 3 weeks. He didn't want to go to church, he spoke constantly of not wanting to go to his friend's houses anymore. He followed me when I left the room and would cry if I left him with his dad while I went to the store.
As much as I could I let him be with me. I held him more and told him every time he mentioned it (up to 100 times a day) that he didn't have to go to his friend's house if he didn't want to, but if he ever did go again mom would always come back to get him. I would never leave him forever. He slowly became his old self. After two weeks he didn't cry when he went to class at church, now I can go to the store by myself without incident.
I would recommend just loving on your son. Let him be with you. If he freaks out when you try to leave (as my son did) then take him with you as much as possible. Give him 10 goodbye hugs whenever you do leave him and tell him a thousand times a day that you will never leave him for forever. He will be his old self again in no time.
God bless you and your family.