In your code, jQuery just looks for the first instance of an input with name q12_3, which in this case has a value of 1. You want an input with name q12_3 that is :checked.
$("#submit").click(() => {
const val = $('input[name=q12_3]:checked').val();
alert(val);
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<table>
<tr>
<td>Sales Promotion</td>
<td><input type="radio" name="q12_3" value="1">1</td>
<td><input type="radio" name="q12_3" value="2">2</td>
<td><input type="radio" name="q12_3" value="3">3</td>
<td><input type="radio" name="q12_3" value="4">4</td>
<td><input type="radio" name="q12_3" value="5">5</td>
</tr>
</table>
<button id="submit">submit</button>
Note that the above code is not the same as using .is(":checked"). jQuery's is() function returns a boolean (true or false) and not (an) element(s).
因为这个答案一直很受关注，所以我还将包括一个普通的JavaScript片段.
document.querySelector("#submit").addEventListener("click", () => {
const val = document.querySelector("input[name=q12_3]:checked").value;
alert(val);
});
<table>
<tr>
<td>Sales Promotion</td>
<td><input type="radio" name="q12_3" value="1">1</td>
<td><input type="radio" name="q12_3" value="2">2</td>
<td><input type="radio" name="q12_3" value="3">3</td>
<td><input type="radio" name="q12_3" value="4">4</td>
<td><input type="radio" name="q12_3" value="5">5</td>
</tr>
</table>
<button id="submit">submit</button>