Basically, these "computer formats" just take the closest noble gas with a lower atomic number than the atom in question and just start writing out the subshells from that point.
In this case, (As) it would be [Ar]4s2 3d10 4p3
The order of the subshells is just in the same order as the periodic table.... K and Ca are 4s, Sc-Zn are 3d, and Ga-Kr are 4p