SOURCE

window.onload = function() {
	var letter = document.getElementById('letter');
	var result = document.getElementById('result');
  var showDiv = document.getElementById('show');

	result.onclick = function() {
		var letterStr = letter.value;
		var lettterArr = letterStr.split(/[\n]/);
    var newArr = lettterArr.map(function(item) {
      return handle(item);
    });
    showDiv.innerHTML = newArr.join('<br />');
	}
}

var upperMin = 65; //大写最小asc
var upperMax = 90; //大写最大asc
var lowerMin = 97; //小写最小asc
var lowerMax = 122; //小写最大asc
//处理函数
function handle(letterStr) {
  var s = [];
  if(!/[a-zA-Z]/.test(letterStr)) {
			alert('请输入大小英文字符');
			return;
		}
		for(var i=0; i<letterStr.length; i++) {
			var _code = letterStr[i].charCodeAt(); //获取每个字符的asc码
			var _move = _code+3; //向后移动3位

			//输入大写且asc大于90 或 输入小写且值大于122，均超出英文字符的范围，都减去23
			if((/[A-Z]/.test(letterStr[i])&&_move > upperMax) || (/[a-z]/.test(letterStr[i])&&_move > lowerMax)) {
				//超出最大asc，减去23
				_move = _code - 23;
			}

			s.push(String.fromCharCode(_move));
		}

		return s.join('');
}

<textarea type="text" id="letter" cols="50" rows="5"></textarea>
<button id="result">转换</button>
<div id="show"></div>