The three key messages from Isaiah 55:1-3 are: 1. God invites all people to come and drink from the waters of salvation, regardless of status or background. 2. Coming to the waters represents finding spiritual fulfillment and eternal life through God, not just physical sustenance. 3. Earthly pursuits and resources cannot fully satisfy or save people in the way that God can. One should listen to God and find true fulfillment through him.