Where can I find Iron, Gold, and Diamond in Minecraft Pocket Edition Survival Mode? I can only find gravel, dirt, wood, sand, and cobblestone.
All of the following information applies to both pocket edition and standard minecraft.
The minecraft wiki has an indepth explanation on where to find each mineral. Your issue is most likely that you have not dug deep enough. As you can see from the graph you need to be less than 60 blocks above the bottom of the map to find ores, and rarer ores are deeper.