I don't know whether or not it's hopeless -- but I do know nothing will change if you don't at least try to overcome the barriers between you. In their present attitude, your children probably won't take the first step -- but you should, and I pray you will.
And one reason you should do this, frankly, is because you are largely responsible for their attitude. I don't know all the facts, of course -- but whatever they were, don't refuse to face your part in what happened. Instead, try to put yourself in the shoes of three little children who not only had to face the trauma of a divorce, but also suddenly discovered that their father didn't even care enough about them to stay in contact. Few things are more hurtful than being rejected by someone we thought loved us.
Have you faced this? Have you realized how self-centered and thoughtless your failure to stay in touch was? Be honest, and let your children know you realize how much you hurt them, and that you need their forgiveness. Even if they refuse, you still need to take this step.
Most of all, my prayer is that you will seek God's forgiveness and turn your life over to Jesus Christ. He alone can forgive your past and change your future. The Bible's promise is true: "If anyone is in Christ, he is a new creation; the old has gone, the new has come!" (2 Corinthians 5:17).