why is the electronic stucture written as 1s 2s 2p 3s 3p 4s. ... ? Shell number 3 can be filled 18 electrons but why isn't 3s 3p 3d written?

Explanation:

The notations for electronic structure of the atom is based upon the application of quantum numbers which specifies the order of filling of atomic orbitals by electrons. There are 4 quantum numbers that include...

Principle QN (n) => specifies location of electron as to major energy level or, ring about the atom of interest.

Orbital QN (l) => specifies the shape of the orbital containing the electron

   s => sphere => holds 2 electrons

   p => figure eight => holds up to 6 electrons

   d => overlapping figure eights => holds up to 10 electrons

   f => multiple overlapping figure eights => holds up to 14 electrons

The order of filling follows the Aufbau Principle that electrons enter the atomic orbital with the lowest available energy content. This order of filling from low energy to higher energies follows the Aufbau Diagram.

This order theoretically can go to infinity, but the scientific community only knows of 7 major energy levels; i.e., the 7 rows of the periodic table. See attached Aufbau diagram and follow the arrows in order from low energy to high energy. Use superscripts for number of electrons equal to atomic number of element of interest.  

H: 1s¹

O: 1s²2s²2p⁴

Al: 1s²2s²2p⁶3s²3p³

Ag: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d⁹

atomic number of carbon is 6

electronic configuration of carbon is 1s²2s²2p²

orbital diagram of c is as shown here in the image.