A finger was a manipulatory appendage on a hand.
Rings were worn on fingers. The First Doctor wore a signet ring on the middle finger of his right hand, (TV: An Unearthly Child) while the Third Doctor wore a ring on the little finger of the same hand. (TV: The Ambassadors of Death) The Master wore rings on the ring finger of both his hands, while the Twelfth Doctor only wore one on his left hand. Missy wore numerous rings on both her middle fingers. (TV: Deep Breath)
The number of fingers on the hand varied among species. The Ralafeans had four thumbs and one finger on each hand and invented the mobile phone before the printing press. (PROSE: Wooden Heart) The Dahensa had two arms which ended in scorpion-like pincers while the other two ended in humanoid fingers. (AUDIO: The Wanderer) Tynakers had four fingerless hands. (PROSE: Losing Track of Time)
Among the many lifeforms with three fingers were the Sontarans, (TV: The Time Warrior) Raxacoricofallapatorians, (TV: Aliens of London) Tractites, (PROSE: Genocide) Anurans, (TV: The Infinite Quest) Kalarians, (PROSE: The Final Sanction) and the Uxarieans. (TV: Colony in Space)
Infiltrating a Sontaran flagship, Donna Noble found a door controlled a switch shaped for the three-fingered Sontaran hand. Reminded by the Tenth Doctor that she had three fingers, Donna molded her five fingers into three to operate the switch. (TV: The Poison Sky)
The Abzorbaloff had hands with three clawed fingers, which exposed him as alien to Ursula Blake and Elton Pope when he attempted to hide behind a copy of The Daily Telegraph newspaper. Ursula was absorbed into the Abzorbaloff's body when his hand grabbed her arm. (TV: Love & Monsters)