February 25, 2017 0351 GMT
Choi Soon-sil, who is facing her own trial over the same scandal, submitted documents to the Constitutional Court on Tuesday, saying she was unable to testify in the case.
Lawmakers voted by a large margin last month to impeach Park over a major corruption scandal, in which Park allegedly colluded to pressure major corporations to contribute money to non-profit foundations controlled by Choi.
Park has denied misconduct in the case but nevertheless apologized for being careless in her ties with Choi.
Two imprisoned former presidential aides who allegedly helped Choi also declined to testify, saying they needed to prepare for their own trials.
Lawmakers, who serve as prosecutors in the trial, allege that it was a stalling tactic by Park's lawyers.
"Three important witnesses all refused to testify, like they planned it ahead. We think there's an invisible hand at work," lawmaker Lee Chun-suak said.
The three will be called as witnesses again next week, the court said, adding it would take steps to forcibly summon them if they once again rejected to appear.
Last week, only one of the four former and current presidential aides the court had summoned testified in the case.
Police are currently trying to locate two of the president's former aides after court employees failed to summon them.
If the Constitutional Court confirms the impeachment vote, a process for which it has 180 days, Park will become the first elected president in the country to be forced out of office.