The first three conferences 
http://www.w3.org/Conferences/Overview-WWW.html
were:

First International Conference on the World-Wide Web
May 25-26-27 1994, CERN, Geneva Switzerland
http://www.cern.ch/WWW94/

WWW Fall '94:  Mosaic and the Web
Chicago, October 17-19 1994
http://archive.ncsa.uiuc.edu/SDG/IT94/Proceedings/WWW2_Proceedings.html

WWW Spring '95
Darmstadt, Germany, April 10-14 1995
http://www.igd.fhg.de/archive/1995_www95/

I need to prove these conferences actually happened - assume webpages
don't count as reliable evidence. Specifically I need to show that
specific papers were published on the dates claimed on them.

Simon Rakov's paper http://archive.ncsa.uiuc.edu/SDG/IT94/Proceedings/Museum/rakov/rakov.html
and the other papers referenced in;
http://www.visi.com/~keefner/pdfs/twwwc.htm 
are the main ones I am interested in.

A source of dated printed proceedings would be ideal, or a reliable
witness, or your other better idea will win the money.

Hello tdaw-ga,

I have found you references to printed matter dealing with these
conferences in the catalog of the British Library.  They all appear in
journals, and all are available on inter-library loan from the British
Library. However, although selected papers plus all abstracts and
workshop reports are published from the first conference, and all
papers from the third, it appears that only a report is available
about the second conference.  Nevertheless, this will serve to prove
the conference actually happened on the dates specified.

Title:  1st World wide web conference. Selected papers, abstracts and
workshop reports
Sponsor:  1st World wide web conference. 1994 May. Geneva, Switzerland
Joint author/editor:  Cailliau. R. Ed 
Series shelving title:  Computer networks and ISDN systems. vol 27 no
2
Subject:  World-Wide Web 
Subject:  Web 
Publisher Name:  Elsevier. 
Date of Publication (and Copyright Date):  1994 
ISSN:  01697552 
Vol. Number:  vol 27 no 2 1994. 
Shelfmark:  3394.117 
 
Title:  Mosaic and the Web. 2nd International World Wide Web
conference. Report
Sponsor:  Mosaic and the Web. 1994 Oct. Chicago, IL 
Series shelving title:  Online and cd rom review. vol 18 no 6 
Subject:  Mosaic 
Subject:  World Wide Web 
Subject:  Web 
Publisher Name:  Learned Information. 
Date of Publication (and Copyright Date):  1994 
ISSN:  13532642 
Vol. Number:  vol 18 no 6 1994. 
Shelfmark:  6260.7605 

Title:  World-wide web. technology, tools and applications. 3rd
International conference. Papers
Sponsor:  World-wide web. 1995 Apr. Darmstadt, Germany 
Joint author/editor:  Kroemker. D. Ed 
Organisation name:  Fraunhofer Institute for Computer Graphics 
Conference name:  Described as proceedings. 
Series shelving title:  Computer networks and ISDN systems. vol 27 no
6
Subject:  world-wide web 
Subject:  tools 
Subject:  technology 
Subject:  WWW 
Subject:  computer graphics 
Publisher Name:  Elsevier. 
Date of Publication (and Copyright Date):  1995 
ISSN:  01697552 
Vol. Number:  vol 27 no 6 1995. 
Shelfmark:  3394.117 
Notes:  Also known as WWW '95

Apologies, I forgot to give you The British Library url.
http://www.bl.uk