What's the difference between an allograph, digraph, and a grapheme? ?

Allograph-.  In linguistics, it's a variant form of a grapheme that is in complementary distribution or free variation with another form of the same grapheme, as " t " and " T " or " n " in " run " and " nn " in " runner ".

Digraph or Digram is a pair of characters used in the orthography of a language to write either a single phoneme, or a sequence of phonemes that does not correspond to the normal values of the two characters combined.

Grapheme is the smallest meaningful contrastive unit in a writing system.

    In linguistics, a grapheme is the smallest functional unit of a writing system. There exists two main opposing grapheme concepts. In the so called referential conception, graphemes are interpreted as the smallest units of writing that correspond with sound.

Screen= S c r ee n

Smell. =  S m e ll

Graph = G r a ph

A digraph is a pair of letters that represent one sound. An allograph is differing letter sequences represent the same phoneme. A grapheme variant can acquire a separate meaning in a specialized writing system.