EDIT: Just noticed you're inquiring about Trello - I would surmise that the same restrictions would apply since Atlassian likes to be consistent, however it can't hurt to shoot them an email and inquire (https://trello.com/contact).
From my experience and research, it depends on which class of products you're purchasing (server or cloud).
Cloud plans are only available in 12 month terms:
Server license renewals can be extended to three years:
(see the Please note section)
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