Answer

In Jainism, tithi means a lunar day in the Jain calendar. If someone says “teen tithi”, it usually means three tithis — यानी three lunar dates on which some observance, restraint, or simple food discipline may be followed. The exact practice can vary by family, sect, and local maryādā. ( jainpedia.org)

Explanation

Sometimes people use “teen tithi” informally to mean:

• observing three specific lunar days in a month or fortnight, or
• referring to a three-day fast.

If they mean the three-day fast, in Jain tradition that is commonly called Attham. ( jainknowledge.com)

Spiritual Understanding

Tithis are important in Jain Dharma because they help remind us to live with restraint, purity, and devotion. On special tithis, many Jains increase swādhyāy, pratikraman, fasting, and self-discipline. ( jainknowledge.com)

Takeaway

So, “teen tithi” is not usually a single fixed Jain technical term. It most likely means three tithis or, in some cases, a three-day fast (Attham).