A witness must be competent to testify:
Here is the Rule on Competency:
(a) General. Every person is competent to be a witness except as otherwise provided in these rules or by statute.
(b) Minimal qualifications. A person is disqualified to testify as a witness if the trial court determines that he:
(1) Lacked the capacity to perceive accurately the matters about which he proposes to testify;
(2) Lacks the capacity to recollect facts;
(3) Lacks the capacity to express himself so as to be understood, either directly or through an interpreter; or
(4) Lacks the capacity to understand the obligation of a witness to tell the truth.
It is up to the judge to determine whether or not the three year old is competent. Three sounds pretty young, but if the child understands the difference between right and wrong and truth and lying then they may be able to testify if they meet the other qualifications.
Case Law: When the competency of an infant to testify is properly raised it is then the duty of the trial court to carefully examine the witness to ascertain whether she (or he) is sufficiently intelligent to observe, recollect and narrate the facts and has a moral sense of obligation to speak the truth. (Quoting Moore v. Commonwealth, Ky., 384 S.W.2d 498, 500 (1964)).
The trial judge will make a ruling pretrial if the child is competent to testify or not. A 3 year old is very young and will be examined by the court, and will decide.
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