Where can I find Iron, Gold, and Diamond in survival mode?

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.

You have to be below sea level to find iron, below level 32 to find gold, and below level 16 to find diamond.