The different plurals are used in different contexts:
**BLYNYDDOEDD** is the standard plural.
Mae hi wedi marw ers blynyddoedd = She's been dead for years.
**BLYNEDD** is the plural used after a number.
Cytundeb dwy flynedd = a two-year contract
**BLWYDD** is used when talking about age
Plentyn tair blwydd oed = a three-year-old child