What is a three stone engagement ring called?

Trilogy is the trade and bench term used by most Australian and UK jewellers, taken from the Greek word for a set of three connected works. It predates the De Beers past, present, future campaign and refers purely to the structural design: a central gem flanked by two significant stones. In the US the same ring is more commonly sold as a past, present, future ring, since De Beers built that name into the American market with the 2001 Trilogy campaign. Trinity ring is a third common name, particularly in religious contexts where the three stones are read as faith, hope and love or as the Holy Trinity.

You will also see plain three-stone ring, anniversary ring (the original use case before De Beers reframed it for engagement) and occasionally triology, which is a marketing misspelling that has stuck on some retailer sites. Jewellers in Australia commonly use trilogy for the basic format and add the cut for clarity, like trilogy oval or trilogy emerald. The structural definition is always the same: one centre stone with two flanking stones large enough to read as significant, not as side accents. Side stones below roughly 0.10ct each push the ring closer to an accented solitaire than a true trilogy.