cheap ready-made seal; inexpensive personal stamp
Refers to a mass-produced, low-cost personal seal (hanko) often sold at stationery shops. The name comes from 三文 (three mon, a small amount of money) and 判 (seal). Used in contrast to a custom-made, registered seal.
See also: 印判
A cheap ready-made seal is enough for urgent documents.
三文判は文房具屋で買える。
You can buy a cheap seal at a stationery shop.