athrawes is feminine, so the feminine form of 'three' would be used. Similarly with 'two' and 'four':
- dau athro, tri athro, pedwar athro
- dwy athrawes, tair athrawes, pedwair athrawes
Although these are not needed at this stage of the course, I have now added a brief note about them in the section notes - people will come across them in everyday Welsh.