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

language = cdata [CI]

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>