I agree with previous post that the boys' actions were quite heroic when they entered the burning church to save the children. I believe they also acted out of a feeling of guilt, since Johnny and Ponyboy caused the fire and put the children's lives in jeopardy when they left their smoldering cigarettes in the church. Johnny, who led the charge into the church, had a poor feeling of self-worth, and he may have felt that the young children's lives were more important than his own. It was gratifying to see Dallas risk his own life, albeit to save Johnny and Pony rather than the kids inside. And it was sad that Johnny, the book's most sympathetic character and whose life had more obstacles than any of the others', had to be the one to die.

This is something that you should answer for yourself because your feelings may well be different from mine.  I will explain my feelings.

To me, what the three greasers did for the kids who were trapped in the fire is rather heroic.  I feel that it is heroic because they put themselves in a very perilous situation when many other people would not have done the same.  I would imagine that the majority of young teens, when faced with a similar situation, would feel that they were unable to be of help.  They might want to help the kids in the church, but they might figure that they were unable to help or they might feel too afraid to try.

I am saddened by the fact that Johnny is so badly injured and I fear for his life as well.

Are Ponyboy, Johnny, and Dallas heroes for saving the kids in the fire?

Those boys were heroes in the sense that they acted selflessly as they went into the fire to help the kids escape.  Selflessness wasn't something they had a lot of practice with, because, through no fault of their own, they spent most of their time and all of their emotional resources protecting and fending for themselves.  Johnny especially was enamored with the idea of the gallant Southern gentleman in Gone With the Wind, and this was the closest he was ever going to get to being the kind of gentleman he so idealized.  Sadly, Johnny's life ended with him reflecting on how much in life he would never experience--while still affirming that going after the kids had been the right thing to do.