VULCAN VENTURES INC, SC 13D, EX-99.3, 2000-06-23

VULCAN VENTURES INC
SC 13D, EX-99.3, 2000-06-23

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<PAGE>   1

                                                                    EXHIBIT 99.3

                             JOINT FILING AGREEMENT


         We, the signatories of the statement to which this Joint Filing
Agreement is attached, hereby agree that such statement is filed, and any
amendments thereto filed by either or both of us will be filed, on behalf of
each of us.

    Dated June 22, 2000.

                             VULCAN VENTURES INCORPORATED



                             By: /s/ William D. Savoy
                                ------------------------------------------------
                                William D. Savoy, Vice President



                                  *
                                ------------------------------------------------
                                Paul G. Allen


                            *By: /s/ William D. Savoy
                                ------------------------------------------------
                                William D. Savoy as Attorney-in Fact for Paul G.
                                Allen pursuant to a Power of Attorney filed on
                                August 30, 1999, with the Schedule 13G of Vulcan
                                Ventures Incorporated and Paul G. Allen for
                                Pathogenesis, Inc. and incorporated herein by
                                reference.