Firstly you can directly get the text using id
selector like this
$('#myId').text();
Second id
attribute's value should not be start with digit. It must be start with character and then it may followed by digit.
The issue with your code is that your $(this)
is basically a window
object not the span
which trigger the click.
So you need to something like this
function GetValue(element) {
var get_text= $(element).next().find("#three").text();
alert(get_text);
}
with HTML
<span id='one' onclick="GetValue(this);">Some value</span>
<span id='two'>Some Span
<span id='three'>Want this text</span>
</span>