Middle three characters

Write a function middleThree(text) that returns the three characters in the middle of the string text: 'NewMumbai' gives 'Mum'.

• text is plain Latin text from 0 to 1000 characters. Every character counts, spaces and signs too, also at the start and the end. Do not trim the text.
• If the length of text is 3 or less, return text as it is. For an empty string return ''.
• If the length is odd and more than 3, return the middle three characters: the same number of characters stays on the left and on the right of them.
• If the length is even and more than 3, there is no single middle. Return null, not ''.

You can cut the part out with slice.