trifocal
Technical term used in optics, photography, and lens design to describe a system with three focal points.
This microscope uses a trifocal lens.
二重焦点 means bifocal (two focal points), while 三重焦点 means trifocal (three focal points).
焦点 is the general term for focal point or focus; 三重焦点 specifies three such points.
Compound of 三重 (さんじゅう, 'triple') and 焦点 (しょうてん, 'focal point').