rannie, liam joel and piper each own a pet. rannie likes to hear his pet sing

If in this question you are asking to correct the grammar in the sentence, I capitalized the first letters of each persons name because they are what is called proper nouns . The names of people places or corporations . For example: Person: John Place: Jamaica Corporation or organization: Cherry County High School , Forever 21 or Amazon.
When you are writing things in a list form in a sentence there has to be a space after each item or thing that you are listing, but before you list the last last thing that you have on your list you should include a conjunction. Conjunctions connect different possibilities or options in a sentence.

Example: "Would you like to eat pizza, rice or sushi?" Or " I would like some strawberries, tomatoes and potatoes from the store".

Adding commas prevent a boring runny sentence like, "Would you like some pizza and rice and sushi?"

Note: It is important that when you are writing that you don't add conjunctions after every word . Its better to list all the items and before you add your last option use a conjunction and end your sentence with your last option.

3. If you notice , after the period the word pet I added a period and capitalized Rannie. Rannie is a proper noun so it is capitalized but whenever you complete a sentence using a period (.) You start your new sentence with a capital letter .

Example: I went to the Amazon yesterday. I found the new iPhone at the store and it was on sale so I bought it. After I bought the phone I went home.