you don't really use them, they are just a name of a share of the upvotes form the @minnowshares account that will last about a month each round.

3 Minnow Shares

These will give upvotes from the @minnowshares account for a limited amount of time and can potentially be traded for an Upvote Share which will give longer term daily upvotes. (For more info see You Got Sponsored !)