Checking string against all string elemenys in an array

Hello,

I have few simple questions. So basically I have a variable that takes user input and I have my array that contains all necessary string values.
I need to run a do until loop that would break only when user input equals any of the array elements (it can only be equal to one of them).

So the questions that arise:
1) how do I run do until loop that would check whether user input equals any of the elements in my array (not just one of them)
2) also that check would have to look at the first three letters of each array element

Thank you guys