linchpin /linchˈ or linshˈpin/

1. A pin used to keep a wheel on its axle
2. A person or thing essential to a plan, organization, etc (figurative)

• three-day eventer
• three-deck
• three-decker
• three-deckers
• three deep
• three-dimensional
• three-dimensionality
• three estates
• three-farthing
• three-farthings
• threefold
• threefoldness
• threefoldnesses
• three-foot
• three-four
• three-four time
• three-halfpence
• three-halfpenny
• three-halfpennyworth
• three-halfpennyworths
• three-handed
• threeha'porth
• three-jaw chuck
• three-jaw chucks
• three kings of Cologne