This is a pictograph of three counting rods. It is the numeral three.
Illustrative Text Reference:
Mohenjo-daro: Tablet: M-1429 A: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3.1: Page 386: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia
Comment:
It is important to distinguish this symbol from the three, full character length, lines (maximum possible light) that comprise the noun summer.
Image Credit:
Indus Script Sign Number 2470: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.