More legal issues for the two men accused of playing a role in the mysterious deaths of three Kansas City Chiefs fans last year ... TMZ Sports has confirmed they've just been sued.

Court records we obtained show Jon Harrington filed a wrongful death lawsuit against Jordan Willis and Ivory Carson in Missouri on Friday ... just days after authorities alleged the two contributed to his son David Harrington's death by supplying him with a lethal mix of drugs at a '24 Chiefs watch party.

The lawsuit states the get-together took place on Jan. 7, 2024 ... at Willis' Platte County home.

Jon claims Willis and Carson gave David cocaine and fentanyl and then "failed to render aid or otherwise attempt to rescue" David when he began suffering from the fatal effects of the drugs.

Jon is asking for unspecified damages.

Willis and Carson were both jailed last week after officials said their actions at the party not only led to David Harrington's death, but also Clayton McGeeney's and Ricky Johnson's.

The men are each facing three charges of involuntary manslaughter and two charges of delivery of a controlled substance. They've both pleaded not guilty to the charges -- and are due back in court this week for another hearing on the matter.