This depends on what arrangement you are using; certain electrodes have different 'names' depending on whether you're using a layout based on a 10-20 system (with e.g. "Cz", "Fz", and similarly named electrodes) or some other layout.

That being said, any paper which reviews the component in question will usually say something about where that component typically appears. For example, any paper that discusses the N400 will also mention that it is typically centro-parietal and broadly distributed, and often maximal near the vertex. You can use those kinds of prose descriptions to figure our where on your particular cap you expect an effect to appear.

For the P300, all electrodes in the parietal lobe (i.e., all those with a name starting with "P" in the 10/20 international system, for example P2, P4, P6, POz...).

The ERN/Ne is maximal at midline frontocentral scalp locations, most typically at the electrode sites FCz and Cz in the 10-20 system, but some also investigate the more anterior site Fz. You may also include an anterior-posterior factor in your statistical analysis (i.e., 3 levels: Fz-FCz-Cz), but it is probably sufficient to focus on the electrode site where your peaks are maximal (e.g., FCz or Cz).