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. (TV: Deep Breath) 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) Tractites, (PROSE: Genocide) Anurans, (TV: The Infinite Quest) Kalarians, (PROSE: The Final Sanction) and the Uxarieans. (TV: Colony in Space)