Former Justice Minister Sabine Leutheusser-Schnarrenberger and Health Minister Daniel Bahr joined Westerwelle and outgoing vice chancellor and Economics Minister Rösler in announcing their resignations on Monday.
Rainer Brüderle automatically lost his position as fraction leader after his party failed to enter parliament for the first time in 65 years following a dismal showing for his party on Sunday's election.
The party, which fell short of the 5 percent hurdle with just 4.8 percent of the vote, lost every one of their 93 seats in parliament. Outside of parliament, the Liberals will struggle to attract attention and are expected to also face financial problems as they will receive much less money from the state.
The leadership resigned amid bitter criticism of their last-minute election campaign strategy of begging for votes from CDU supporters, which many supporters felt meant the party lost the respect of the electorate.
Meanwhile all hopes were pinned on emerging leader 34-year-old Christian Linder, who has said he will steer the party back into parliament by the next election.
To do this, the party will have to regain respect, said Linder and suggested that the party had neglected to highlight where it differed from its coalition partners the CDU.
"Not everything was wrong, but a lot of it was obviously not convincing," he said.
The party is still represented in nine of 16 regional parliaments and scraped into the Hesse regional parliament in Sunday's vote with exactly 5 percent of the vote. The Liberals are now only in regional Saxon government, where they rule in a coalition with the CDU.
The FDP will elect their new leadership at their party conference in November.