"Three boys were killed yesterday."
Translation:Gaeth tri bachgen eu lladd ddoe.
This is explained in the notes for the earlier section on 'Numbers'.
Welsh uses the singular noun after numbers, not the plural:
- bachgen (bechgyn) - boy (boys)
- tri/pedwar/pum bachgen - three/four/five boys
- dau fachgen - two boys (soft mutation after dau/dwy)
I’m not quite sure what you by ‘it changed...’, but both caeth and gaeth are acceptable in the answers here. Perhaps you made some other mistake? If you used gaeth and hogyn the answer would be:
- Gaeth tri hogyn eu lladd ddoe.