My laptop just did what I feared: It created three footnote columns in a new blank document. But it still wouldn't do that in the document where they belong, so here's my plan.
I'll create a new blank document where the computer will arrange footnotes into three columns when it can. Then I'll copy text from the other document, paste it into the new one and add footnotes to what I've just pasted.
To paraphrase Patrick Henry, give me Knuth's TeX or give death.