(1) The Commonwealth has the burden of proving every element of the case beyond a reasonable doubt, except as provided in subsection (3). This provision, however, does not require disproof of any element that is entitled a "defense," as that term is used in this code, unless the evidence tending to support the defense is of such probative force that in the absence of countervailing evidence the defendant would be entitled to a directed verdict of acquittal.
(2) No court can require notice of a defense prior to trial time.
(3) The defendant has the burden of proving an element of a case only if the statute which contains that element provides that the defendant may prove such element in exculpation of his conduct.