Question about OneHR Reference Check; Not Enough Supervisors?

Hi! I applied for an UN organization months ago, got interviewed, and is now in progress of reference check by OneHR.

However, there's one issue: the organization I'm applying for asked for at least 3 referees and within those referees they want at least 2 supervisors, but OneHR staff emailed me directly, asking for at least 3 supervisors.

It wouldn't be a problem if I could find the last supervisor easily... One of my previous bosses was very abusive and I can't put their name there: they tried to bully me, threatened me, and accused me of something that I didn't do for a year. And that boss' boss was almost never involved in my day-to-day operation and I'm not really sure if I can ask them to write anything for me given our non-existent relationship.

Have anyone experienced something like this? Would they accept colleagues who were more senior than myself like a team leader (though leaders aren't exactly managers)? Or maybe my understanding of "supervisors" is wrong?

I don't know what to do to be honest so I appreciate all advice! Also, I can't give you more information about the job or the organization because I don't wish to be identified...