The last time the Mets gave up 9 or more runs three games in a row, before these past three games, was June 21 to 23, 2001 (starter Kevin Appier was terrible, but starters Rick Reed and Glendon Rusch were betrayed by the bullpen -- especially egregious was Rick White giving up 6 runs in the 11th inning against Atlanta). The Mets did not have any such streak of 9 or more runs surrendered in three consecutive games in the 1990s, had only one such streak in the 1980s and only one in the 1970s. In their very early expansions years, 1962 to 1965, the Mets had four streaks of three straight games giving up 9 or more runs. The Mets have never given up 9 or more in four straight games.
The longest streak by any team since 1962, when the Mets came into the league, of games giving up 9 or more runs is by the Mariners, who had a seven game streak of such games in 2000. Both the White Sox and Astros have had four-game streaks surrendering 9 or more runs this season.
(source as usual is the baseball-reference.com Play Index)