Please don't be too worried . As long as you tell him in a kind and simple way , it will be fine. What matters to any three year old is who loves him and cares for him - that's you, you are his real dad. That other man is just his biological dad.
Just as a PP said up thread - something like this :
Mum A and Dad B were Boy friend and girl friend . They hadn't known each other for very long when they discovered that they were going to have a baby. This was a big suprise, mum A was very happy but dad B wasn't. He didn't want to be a a dad, he wanted to do X Y and Z.
So he left and went to do X. MUm A was a bit sad and cross that he had left but she was very happy that she was going to have a baby . GRan C and grandpa D were happy too, they bought a buggy for the baby .
Gran E and AUntie F knitted the baby some clothes . Uncle F painted a room for the baby . Everyone in the family was so excited and happy they couldn't wait to see the baby . Then you were born .
[ lots of nice stuff here about how happy everyone was , what baby looked like, what names mum thought about and why she chose the ones she did . Lost of cute details here ]
Then add how mum A met Dad B and how he is dad now.
[ how happy you are to be his dad , want you do together etc, stories about fun times you have had, birthdays, holidays etc ]
Remember Your son is much more interested in what toys GRan bought him or what colour his hair was when he was born than he is in some aspect of biology that is currently meaningless to him.
But you have told him the facts and when he is older he can ask more when he wants to . Which probably won't be for years .
Please don't ever lie to him about it or let anyone else lie to him. Just try to answer in a kind way . I'm guessing that his bio dad left befroe he was born, so it's fine to say that he didn't want to be a dad then, maybe he was too young or he didn't love mum and didn't want to stay with her.
He left for reasons to do with HIm and what was going on in his head and his life and NOTHING to do with the baby .
Be prepared for him to ask questions about where he is now.
Please don't ever tell him that his dad wanted him aborted . It's not necessary and it's unkind. There's a difference between being honest and telling the awful details .