sensible, sensitive, and sentient
Solution 1:
Where did you find definitions that show the words to be synonyms? They are hardly even close:
sen·tient
- Having sense perception; conscious: "The living knew themselves just sentient puppets on God's stage" (T.E. Lawrence).
- Experiencing sensation or feeling.
sen·si·tive
- Capable of perceiving with a sense or senses.
- Responsive to external conditions or stimulation.
- Susceptible to the attitudes, feelings, or circumstances of others.
- Quick to take offense; touchy.
- Easily irritated: sensitive skin.
- Readily altered by the action of an agent: film that is sensitive to light.
- Registering very slight differences or changes of condition. Used of an instrument.
- Fluctuating or tending to fluctuate, as in price: sensitive stocks.
- Of or relating to classified information: sensitive defense data; holds a sensitive position in the State Department.
sen·si·ble
- Perceptible by the senses or by the mind.
- Readily perceived; appreciable.
- Having the faculty of sensation; able to feel or perceive.
- Having a perception of something; cognizant: "I am sensible that a good deal more is still to be done" (Edmund Burke). See Synonyms at aware.
- Acting with or exhibiting good sense: a sensible person; a sensible choice.
In everyday usage, sentient means conscious or self-aware (in the sci-fi world, it is often used to differentiate intelligent life forms from animals). Sensitive means being receptive to stimuli/ For example, I have sensitive eyes and bright lights bother me. Sensible means reasonable, possessing common sense.
The three words share common origins but do not really have similar, let alone identical, meanings.
Solution 2:
To steal your method for defining, I would say:
"sentient" is used to describe a person who has the capacity to perceive and understand.
Oxford's definition - able to perceive or feel things