The chemical formula 3C6H8O7 represents citric acid. Here's how to calculate the number of atoms:

* C (Carbon): 3 * 6 = 18 atoms

* H (Hydrogen): 3 * 8 = 24 atoms

* O (Oxygen): 3 * 7 = 21 atoms

Total: 18 + 24 + 21 = 63 atoms

Therefore, there are 63 atoms in 3C6H8O7 (citric acid).