<input type="text" class="abc" placeHolder="Name">
<input type="text" class="abc" placeHolder="Surname">
<input type="text" class="abc" placeHolder="Address">
<button id="btn">Show</button>

const button = document.getElementById("btn");
var inputs = document.getElementsByClassName("abc");
button.addEventListener('click', function(){
   for (var i = 0; i <= inputs.lenght; i++){
        alert(inputs[i].value);
}
});