The basic education received in primary schools, especially (but not necessarily limited to) reading, writing and arithmetic.
1875, Louisa M[ay] Alcott, chapter 8, in Eight Cousins:
“You shall teach me, and when I am a woman we will set up a school where nothing but the three R's shall be taught, and all the children live on oatmeal, and the girls have waists a yard round,” said Rose, with a sudden saucy smile dimpling her cheeks.