I have just recently turned 14. I am 5 foot 3 and I have only grown 3 inches in two years. I have always been average height for my age, but now I'm nearly one of the smallest boys in the year, and I hate it! Nearly all the girls are taller than me. I used to be one of the best in my year at sport, but now I'm no way near as strong or as tall as other boys and don't make the sports teams like I used to. I have some pubic hair around the base of penis and have lost a lot of body fat in the past year, but I really want to know when I will grow! I pray to the Lord to hurry up my growth, but I know He can't change my genetics as he created me. My older brother grew when he 15.5, but I couldn't stand another year like this! Please help me and tell me when you think I will grow.
Based on the few things that you've mentioned, you are somewhere in stage 2 of development. You can use the Tanner Stage Calculator to get a more detailed estimate. A growth spurt happens during stage 3.
Based on your brother, I must assume that you are from a lineage that develops later than usual. Since the average time for a growth spurt among all males is around age 13.5, it is natural that over half of the boys you know are further ahead in their development than you are. The average age for a growth spurt in girls is around age 11.5, so the majority of the girls you know will have gotten taller as well.
While I know you don't like it, there is nothing that can be done about it. The body develops at time and rate that is best for it. It has long been known that trying to mess with the body's development causes unintended and unforeseen consequences that are not good for the person. God knows what He was doing when He designed people and messing with it when there is nothing wrong isn't a good thing.
What you can do is learn patience -- I know, something you don't want to hear. "My brethren, count it all joy when you fall into various trials, knowing that the testing of your faith produces patience. But let patience have its perfect work, that you may be perfect and complete, lacking nothing" (James 1:2-4). You may not be first to reach an adult body among your peers, but you will eventually get there.
Is God punishing me for watching porn or something?
No, delayed growth is not a punishment. If God punished everyone for looking at porn by slowing down their growth, then I would suspect that most of your peers would still be waiting.
However, it isn't good for you spiritually to be involved in pornography. It warps your thinking and corrupts your behavior. See: Lies Pornography Tells Men.