Probably the file is not in the path specified. '../../../' will move 3 step up to the directory in which the page is located and look for the js file in a folder named JS.
Also the language attribute is Deprecated.
See Scripts:
18.2.1 The SCRIPT element
Deprecated. This attribute specifies the scripting language of the contents of this element. Its value is an identifier for the language, but since these identifiers are not standard, this attribute has been deprecated in favor of type.
Edit
Try changing
<script src="../../../JS/Registration.js" language="javascript" type="text/javascript" />
to
<script src="../../../JS/Registration.js" language="javascript" type="text/javascript"></script>