How to remove first 3 letters in JQuery?

No jQuery needed.

"cat1234".slice(3);

or

"cat1234".substring(3);

or

"cat1234".substr(3);

var str="cat1234";
console.log("using slice =",str.slice(3));
console.log("using substring =",str.substring(3));
console.log("using substr =",str.substr(3));

var val = 'cat1234';
var newVal = val.substring(3, val.length);

You don't need jQuery to do this, JavaScript will do:

"cat1235".substring(3) // yields 1235