It is not a 3 day extension.
You might want to read the PA Supreme Court ruling.
"Based on our disposition of all of the claims set forth above, we grant relief on the claims set forth in Counts I, II, and V of the DEMOCRAT Party’s petition for review as follows and hold that: (Count I) the Election Code permits county boards of election to collect hand-delivered mail-in ballots at locations other than their office addresses including drop-boxes as indicated herein, see supra. at 20 n. 15; (Count II) a three-day extension of the absentee and mail-in ballot received-by deadline is adopted such that 35 Respondent has not asserted that the Pennsylvania Constitution offers greater protection under the circumstances presented. Thus, for purposes of our review, we treat them as co-extensive. [J-96-2020] - 63 ballots mailed by voters via the United States Postal Service and postmarked by 8:00 p.m. on Election Day , November 3, 2020, shall be counted if they are otherwise valid and received by the county boards of election on or before 5:00 p.m. on November 6, 2020;ballots received within this period that lack a postmark or other proof of mailing, or for which the postmark or other proof of mailing is illegible, will be presumed to have been mailed by Election Day"
https://www.scotusblog.com/wp-content/uploads/2020/09/1589ec07-66e0-485c-b9be-9f7a03e99643-1.pdf