"One, two, three."
Translation:Uno, dos, tres.
This is the usage:
Uno is shortened to "un" when used as an article or numeral adjective and before a masculine singular noun or generally immediately before a feminine singular noun beginning with a tonic "a" or "ha", e.g. un alma pura, un hacha antigua.
Similarly, other numbers (considered adjectives before a noun) ending in 1, e.g. *(21) veintiún hombres, (41) cuarenta y un animales, (101) ciento un águilas.
"Uno" is the number one, used when counting. "Un" is the modification of "uno" as an adjective before masculine nouns (also means "a/an"). (Ejemplo: un perro) When it's before feminine nouns, it's "una". (Ejemplo: una chica) "Veintiuno" would change to "veintiún" when put before masculine nouns. "Veintiuno" would change to "veintiuna" when put before feminine nouns. Do you understand this? Did I make it clear enough? Great question, by the way!