Only Three Words?
All art is just a snapshot in time. But a three line haiku seems an extreme. Participation is the key to reading or listening to haiku. It is meant to expand images as it is repeated.
SUMMER: Recalls images of warmth, sunshine, happy days, beach, picnics, sandals and much more.
WATERMELON: Satisfies thirst on a hot summer day. Seed spitting contests. Picnics and perhaps a few pesky flies.
TOGETHER: Summer is a get together time. Family and friends and the pleasure of eating, laughing and talking together.
So you can see with these three words many thoughts can arise. The haiku poet may have one thought in mind, while it is up to the reader to embellish from his/her own memories. You then become a haiku enthusiast and might be tempted to compose a few yourself.