Three Sisters, the painting, is obviously three mountains.
Less obviously it is about sisterhood, women standing firm and tall when they stand together.
Less obviously it is also an homage to the Three Sisters method of planting. This method is more nutritious to the soil per square acre than modern industrial farming.
[from wikipedia: The Three Sisters are the three main agricultural crops of various Indigenous peoples of North America: winter squash, maize (corn), and climbing beans (typically tepary beans or common beans). Originating in Mesoamerica, these three crops were carried northward, up the river valleys over generations, far afield to the Mandan and Iroquois who, among others, ...