Their are three kids in the room.
What is your question?
Perhaps you are asking weather the sentence is grammatically correct? If so, your use of "their" is incorrect.
This is what you mean:
There are three kids in the room.
To determine how many kids are in the room, you can count them. Start by visualizing the room in your mind and imagine each child present. Then start counting, making sure to count each child only once. Once you have counted all the children, you will have the answer to the question.
Since you mentioned there are three kids in the room, you don't have to perform the counting process since you already have the answer. Therefore, there are three kids in the room.