expression
afternoon snack; afternoon tea
A light meal or snack taken around 3 p.m., often sweet, similar to British afternoon tea. The phrase is a set expression.
３時のおやつにクッキーを食べた。
I had cookies for my 3 p.m. snack.
今日の３時のおやつはケーキです。
Today's afternoon snack is cake.
おやつ is a general term for a snack between meals, not necessarily at 3 p.m. ３時のおやつ specifically refers to the traditional mid-afternoon snack time.
From ３時 (3 o'clock) + おやつ (snack). The phrase reflects the Japanese custom of having a snack around 3 p.m., similar to the concept of afternoon tea.