Exhibit 24.2

                               POWER OF ATTORNEY

        The undersigned constitutes and appoints Joel Rubinstein, Henrikki
Harsu, Adam Berkaw, Fangzhou Ying and Christina Min, or any of them acting
singly, as the undersigned's true and lawful attorneys-in-fact and agents, with
full power of substitution and resubstitution, for the undersigned and in the
undersigned's name, place and stead, to sign any and all Securities and Exchange
Commission ("SEC") statements of beneficial ownership of securities of dMY
Technology Group, Inc. III (the "Company") on Schedule 13D as required under
Section 13 and Forms 3, 4 and 5 as required under Section 16(a) of the
Securities Exchange Act of 1934, as amended, and any amendments thereto, and to
file the same with all exhibits thereto, and other documents in connection
therewith, with the SEC, the Company and any stock exchange on which any of the
Company's securities are listed, granting unto said attorneys-in-fact and
agents, and each of them, full power and authority to do and perform each act
and thing requisite and necessary to be done under said Section 13 and Section
16(a), as fully and to all intents and purposes as the undersigned might or
could do in person, hereby ratifying and confirming all that said attorneys-in-
fact and agents, and each of them, may lawfully do or cause to be done by virtue
hereof.

        A copy of this power of attorney shall be filed with the SEC.  The
authorization set forth above shall continue in full force and effect until the
undersigned revokes such authorization by written instructions to the attorneys-
in-fact.

        The authority granted hereby shall in no event be deemed to impose or
create any duty on behalf of the attorneys-in-fact with respect to the
undersigned's obligations to file a Form ID, Schedule 13Ds and Forms 3, 4 and 5
with the SEC.


Dated: September 14, 2020

                                             /s/ Harry L. You
                                             -----------------------------------
                                             Harry L. You