A trigram may also refer to Ba gua, a philosophical concept in ancient China. It may also refer to a three-letter acronym

Three-letter acronym

A three-letter acronym, three-letter abbreviation, or TLA is an abbreviation, specifically an acronym, alphabetism, or initialism, consisting of three letters...

.

Trigrams are a special case of the N-gram

N-gram

In the fields of computational linguistics and probability, an n-gram is a contiguous sequence of n items from a given sequence of text or speech. The items in question can be phonemes, syllables, letters, words or base pairs according to the application...

, where N is 3. They are often used in natural language processing

Natural language processing

Natural language processing is a field of computer science and linguistics concerned with the interactions between computers and human languages; it began as a branch of artificial intelligence....

for doing statistical analysis of texts.

Frequency

The 16 most common trigrams in English are:

Rank	Trigram
1	the
2	and
3	tha
4	ent
5	ing
6	ion
7	tio
8	for
9	nde
10	has
11	nce
12	edt
13	tis
14	oft
15	sth
16	men

Examples

The sentence "the quick red fox jumps over the lazy brown dog" has the following word level trigrams:

the quick red
quick red fox
red fox jumps
fox jumps over
jumps over the
over the lazy
the lazy brown
lazy brown dog

And the word-level trigram "the quick red" has the following character-level trigrams (where an underscore "_" marks a space):
the qui k_r
he_ uic _re
e_q ick red
_qu ck_