There are at least 4,545 English words that contain the letter X. Here are a few of the longer words.
18-letter words
deoxyribonucleases, dextroamphetamines, exceptionabilities, expressionlessness, extemporaneousness, exterritorialities, gedankenexperiment, hydroxytryptamines, inexhaustibilities, inexplicablenesses, inexpressibilities, inexpressivenesses, inexpugnablenesses, overextrapolations
19-letter words
deoxyribonucleotide, exhibitionistically, expressionistically, extralinguistically, extraordinarinesses, extraterritoriality, gedankenexperiments, hyperexcitabilities, inexhaustiblenesses, inexpressiblenesses, unexceptionableness
20-letter words
deoxyribonucleotides, existentialistically, expressionlessnesses, extemporaneousnesses
21-letter words
extraterritorialities, unexceptionablenesses
22-letter words
carboxymethylcellulose, hexamethylenetetramine
23-letter words
carboxymethylcelluloses, hexamethylenetetramines
THere are 303 words that begin with the letter X.
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern --X-ALL. That is, seven letter words with 3rd letter X and 5th letter A and 6th letter L and 7th letter L. In alphabetical order, they are: boxball
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern ---ALL-X. That is, eight letter words with 4th letter A and 5th letter L and 6th letter L and 8th letter X. In alphabetical order, they are: parallax
Some words that have the letter X in the middle are:asexualaxeboxerbuxomcoaxingcoexistDixieepoxiedfixedflexionhexedhoaxinglexismaximmixedmoxienexuspixelpixieproxiessexessixthsixtytaxedTexastoxictoxinvexedvixenwaxen
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern X---X---. That is, eight letter words with 1st letter X and 5th letter X. In alphabetical order, they are: xeroxing
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 3 words with the pattern ---X-ALL-. That is, nine letter words with 4th letter X and 6th letter A and 7th letter L and 8th letter L. In alphabetical order, they are: asexually biaxially coaxially
There is no double letter word with the letter x
words: x-ray
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern --X---XI-. That is, nine letter words with 3rd letter X and 7th letter X and 8th letter I. In alphabetical order, they are: saxitoxin
You are gonna hate me, but look in the dictionary.
X-rayXenonXeroxSince the question asked for "three-letter" words beginning with "X", how about "xis"?
7 letter word 3rd letter x