<PAGE>
Prospectus Supplement filed under Rule 424(b)(3)
in connection with Registration Statement No. 333-48047
PROSPECTUS SUPPLEMENT NO. 3 DATED SEPTEMBER 18, 1998
(To Prospectus Dated May 28, 1998)
PLATINUM technology, inc.
$150,000,000 6.25% Convertible Subordinated Notes Due 2002
and Shares of Common Stock Issuable Upon Conversion Thereof
This Prospectus Supplement No. 3 ("Third Prospectus Supplement") supplements
information contained in that certain Prospectus, dated May 28, 1998 (the
"Prospectus") as supplemented by Prospectus Supplement No. 1 dated July 13, 1998
("First Prospectus Supplement") and Prospectus Supplement No. 2 dated August 31,
1998 ("Second Prospectus Supplement"), relating to the offer and sale by certain
Selling Securityholders of up to $150,000,000 6.25% Convertible Subordinated
Notes Due 2002 and shares of Common Stock issuable upon conversion thereof (the
"Securities") of PLATINUM technology, inc. (the "Company"). This Third
Prospectus Supplement is not complete without, and may not be delivered or
utilized except in connection with, the Prospectus, the First Prospectus
Supplement and the Second Prospectus Supplement, including any amendments or
supplements thereto. Capitalized terms used herein but not defined have the
meanings assigned to such terms in the Prospectus. The following table amends
and supplements the information set forth in the Prospectus under the caption
"Selling Securityholders," and, with respect to "Deutsche Bank Securities Inc.",
the information below supersedes the information contained in the Prospectus for
"Deutsche Morgan Grenfell Inc." in the table under the caption "Selling
Securityholders."
<TABLE>
<CAPTION>
Principal Amount of
Securities Beneficially Owned Number of
----------------------------------------- Shares of
Number of Principal Common
Principal Shares of Amount of Stock
Amount of Common Notes That That May
Name Notes Percentage Stock(1)(2) May Be Sold Be Sold
- --------------------------------- ------------ ---------- ----------- ----------- ---------
<S> <C> <C> <C> <C> <C>
Banc Boston Robertson Stephens $ 1,500,000 1.0% 41,605 $ 1,500,000 41,605
Deutsche Bank Securities Inc. (3) 6,730,000 4.5 186,672 6,730,000 186,672
Montaigne Fund N.V. 100,000 * 2,773 100,000 2,773
Tocqueville N.V. 100,000 * 2,773 100,000 2,773
Any other holders of Notes(4)(5) 19,295,000 12.9 535,191 19,295,000 535,191
</TABLE>
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* Less than 1%.
(1) Assumes conversion of the full amount of Notes held by such holder at the
initial rate of $36.0525 in principal amount of Notes per share of Common
Stock; such conversion price is subject to adjustment as described in the
Prospectus under "Description of Notes - Conversion." Under the terms of
the Indenture, fractional shares will not be issued upon conversion of the
Notes; cash will be paid in lieu of any fractional shares.
(2) The number of conversion shares beneficially owned by each Selling
Securityholder named herein is less than 1% of the Company's outstanding
Common Stock as of September 18, 1998.
(3) Deutsche Bank Securities Inc. served as the sole placement agent for the
December 1997 offering of the Notes.
(4) Information concerning other Selling Securityholders will be set forth in
supplements to this Prospectus from time to time, if required.
(5) Assumes that "Any other holders of Notes" do not beneficially own any Common
Stock other than the Common Stock issuable upon conversion of the Notes at
the initial conversion price.
This Prospectus Supplement is dated September 18, 1998
