Uhmmm... he's 3! Just let it go for now. Most kids I know that age don't like the water. If you force the issue, he'll just resist more and probably end up hating the water more. Step away for a few months, then try again. Make it fun for him and let it be his decision, and he'll be more likely to want to be in the water. My oldest hated water from age 2-4 and wouldn't go near a pool. So we stopped mentioning it and just 'gave up' temporairly. Less than 4 months after we stopped pushing, he was asking to go to the neighborhood pool and just jumping in fearlessly on his own -- that is when we started lessons with him. He's 7 now and part fish during the summer.
The whole point of preschool "swim" lessons is just to get them familiar with the water and not be afraid of it. Most experts agree that most children (not all, there are always exceptions and I know a few of them!) lack the physical and mental coordination to really swim until closer to age 5. Yes, preschoolers can learn to 'swim' a little bit and stay afloat for a few minutes... but they are not truly 'safe' in the water until a little older.
The best way for a child to be safe in the water is to simply wear a life jacket and have adults nearby. My younger kids wear a life jacket everytime we're near the water, even though the 4 year old can 'swim' a little bit. My 7 year old still wears it sometimes... he knows his limitations and when he starts to get tired he puts it on.
I don't understand the push to get kids swimming early. When we were kids, it was unheard of until kids were in school - and even then it was mostly just splashing around having fun. Most kids didn't take real swim lessons until almost they were pre-teens, if at all! Whatever happened to just going to the pool to have fun with your family and learning that way?