Doing some minor work to lighting in a 1957 house. The circuit seems modern(ish) but has 3 wires plus earth (instead of 2 plus earth) coloured yellow, red and blue. Need assistance understanding circuit and adding lights.

I can tell by your use of the word "earth" that you might be in Europe, UK most likely. I am only familiar with color codes frm North America. I am not sure if there is anyone else here that knows your color code.

Perhaps,if you have a volt meter, for 300 volta AC, then you can test each wire to determine which is hot and which is neutral, if applicable.

Have you done a search of UK websites looking for your color code?