5 save 2 bytes
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HTML+Javascript, 73 6260 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

Suggestions from the comments take this further:

<body onload=open('',name=0),setTimeoutname=setTimeout(close,3e3))>Hello world

HTML+Javascript, 73 62 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

Suggestions from the comments take this further:

<body onload=open('',name=0),setTimeout(close,3e3)>Hello world

HTML+Javascript, 73 60 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

Suggestions from the comments take this further:

<body onload=open('',name=setTimeout(close,3e3))>Hello world
4 added 142 characters in body
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HTML+Javascript, 7373 62 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

Suggestions from the comments take this further:

<body onload=open('',name=0),setTimeout(close,3e3)>Hello world

HTML+Javascript, 73 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

HTML+Javascript, 73 62 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

Suggestions from the comments take this further:

<body onload=open('',name=0),setTimeout(close,3e3)>Hello world
3 Added code colorization
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HTML+Javascript, 73 characters

HTML+Javascript, 73 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world
<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

HTML+Javascript, 73 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

HTML+Javascript, 73 characters

<script>setTimeout("open('','_self','');close()",3e3)</script>Hello world

This works in Chrome, but may not be portable to other browsers.

2 deleted 23 characters in body
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1
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