Tercentenary
/ˌtɜːrsenˈtenəri/
Definition 1 of 2
Pro Tip 1/3Event Specific
Use 'tercentenary' when referring to 300 years since a particular event, not general time spans. 
The university is planning a conference for its tercentenary.
Pro Tip 2/3Celebratory Context
Often used when planning or reporting celebrations of 300-year milestones. 
The tercentenary of the town's founding included a time capsule burial.
Pro Tip 3/3Formal Occasions
Best suited for formal or official communications and writings. 
A tercentenary medal was commissioned for the occasion.