Showing or hiding groups of three text fields
What problem are you trying to solve? I've left your code doing the same things but rather than interrogate the DOM each time I've cached everything in local variables. 

// Execute code in an Immediately Invoked Function Expression (IIFE)
// This helps stop your variables polluting the global scope.
(function () { 

    // opt in to ECMAScript 5's strict mode 
    "use strict";

    // As we aren't changing the DOM just store all of the selectors in 
    // local variables - it's cheaper than hitting the DOM each time.
    var inputRow1 = $('#inputRow1').hide(), 
        inputRow2 = $('#inputRow2').hide(),
        addRowLink = $('#add-more'),
        removeRowLink = $("#remove-last").hide(),
        // rather than checking the DOM for number of hidden children,
        // just keep track of the last row visible (0, 1 or 2)
        lastRowVisible = 0; 


    addRowLink.click(function(event) {
        // stop the default action of the click event.
        event.preventDefault();

        // This is the initial state with only the first 3 input boxes showing
        // we need to show row 1 and the remove last link as well as increment the
        // last row visible to 1.
        if (lastRowVisible === 0) {
            removeRowLink.show();
            inputRow1.show();
            ++lastRowVisible;
        // The first two rows are visible. Show the last row and hide the add-more
        // link as well as increment the last row visible to 2.
        } else if (lastRowVisible === 1) {
            addRowLink.hide();
            inputRow2.show();
            ++lastRowVisible;
        }   
    });

    removeRowLink.click(function(event){
        // stop the default action of the click event.
        event.preventDefault();

        // All 3 rows currently visible: need to hide the last row, show
        // the add-more link and decrement the last row visible to 1.
        if (lastRowVisible === 2) {
            addRowLink.show();
            inputRow2.hide();
            --lastRowVisible;

        // 2 rows currently visible: need to hide the second row and the
        // remove-last link and decrement the last row visible to 0.
        } else if (lastRowVisible === 1) {
            removeRowLink.hide();
            inputRow1.hide();
            --lastRowVisible;
        }   
    });   
}());


Here's a working fiddle: http://jsfiddle.net/RobH/CDVBs/1/

Have you considered a solution that starts with only 3 inputs and then creates extra inputs instead of showing ones that are already declared but hidden?