How can glucose fructose and galactose can be different molecules even though they all have the same molecular formula?

They are isomers of each other. It's just the arrangement of atoms that's different (glucose is an aldose, while fructose is a ketose).

The structural formula is differrent. Never forget that one form of Thalidomide is beneficial. The optical isomer caused birth defects.

Isomers

isomers

They are isomers of each other. It's just the arrangement of atoms that's different (glucose is an aldose, while fructose is a ketose).