三年级上册用英语介绍自己喜欢的动物，可以用以下六句话：
I like dogs.
They are cute animals.
They have two smart ears, a long nose, and big eyes.
They are human's good friends.
They are our door-looker.
They will bite those people who are thieves.
或者可以尝试以下句子：
My favorite animal is a dog.
Dogs are soft and cuddly.
They have a tail and four legs.
They run around all the time.
Cats are like a smaller version of tigers.
They like to eat fruits and vegetables.
选择哪种方式取决于你希望更详细地描述动物的特征，还是简单地表达你对动物的喜爱。希望这些句子能帮助你用英语介绍自己喜欢的动物。