What are the three rules of irregular nouns?

There aren't really "three rules" for irregular nouns. The term "irregular" itself means they don't follow any consistent rules. They just have to be memorized.

However, there are some patterns you can look for that might make memorization easier:

1. Many irregular plurals are formed by adding "-en" or "-ren"

* child - children

* ox - oxen

* woman - women

* brother - brethren

2. Some irregular plurals change the vowel sound:

* tooth - teeth

* foot - feet

* mouse - mice

3. Some irregular nouns have the same form for singular and plural:

* sheep - sheep

* deer - deer

* fish - fish

The best way to learn irregular nouns is to study lists and practice using them in sentences.