I want to say "The letter was trifolded" but all my spelling sources don't recognize "trifolded" as a word. Trifold is a word, of course. Is my intended usage incorrect? If so, how would I say it instead?
Perfectly good word, as a participial modifier or a passive-voice verb:
From Always Say Goodbye: A Lew Fonesca Mystery by Stuart Kaminsky:
His fists were clenched. The hog was led out of the ring with people shouting, "Take care now, Fred!" and "Good job, Fred!" Lew reached into his back pocket, pulled out the trifolded papers with their blue cover sheet and handed them to Borg.
From Only Time Will Tell by Suzanne Hoos:
She caught her reflection in the trifolded mirror. She looked pale, beaten. Thirty-three years old and she should be hitting her stride. Instead, she chose to giveup.
From Current Therapy in Reptile Medicine and Surgery, D. R. Mader and S. J. Divers, eds.:
A pharmaceutical fold can be created with a piece of clean paper that is trifolded from top to bottom, then trifolded again from side to side. The paper is unfolded and the collected item is placed in the center; the paper is then refolded, which secures the evidence.
While it doesn't seem to have a universal definition, "Letter Fold" seems to be an industry standard for what you are describing.
This particular supplier defines Tri-fold (or trifold) slightly differently from letter fold.
Yes, 'trifolded' is a verb, with both transitive and intransitive uses. In the simple past tense,
He trifolded the letter
means 'he folded the letter into three parts' (the parts needn't be thirds, and in my experience rarely are thirds). This is a transitive use of the verb.
The letter was trifolded
means 'the letter was folded into three parts'.
Most or all dictionaries do not currently show 'trifold' as a verb, and most or all spell-checkers will complain about 'trifolded', but that need not deter you. As others have pointed out in the comments and answers, 'trifolded' is in use (deadrat exampled some of those uses in an answer) and 'trifolded' communicates what you intend (which Kristina Lopez stipulated in a comment).
'Trifolded', used in the way you use it, is also conventional: its conversion from adjective to intransitive verb observes a linguistically conventional process, and that process adds additional, conventional senses to the meaning of the word.
The process of that conversion from adjective to intransitive verb in the case of 'trifolded' is somewhat complex. As explained in an article from Translation Journal called "Grammatical Conversion in English: Some new trends in lexical evolution", the primary conversion is from adjective to transitive verb:
Adjectives can also go through the process of conversion, especially to verbs. De-adjectival verbs get the meaning of "to make (adjective)". It can be easily seen by means of examples like 'black(ed)' (45) (to make black), ....
The secondary conversion is from transitive to intransitive verb:
Transitive verbs can also be used intransitively, that is the case of 'closed' (84). This category has been previously converted from adjective to verb, and, afterwards, it has experienced a secondary conversion from transitive to intransitive verb. In this sense, the verb would change the meaning from "to make close" (85) (transitive use) to "to become closed" (intransitive use) (84).