Three syllables: HAL-see-un
"Hal" rhymes with "pal" and "gal."
Stress goes on the first syllable. The accent on the other two syllables is about equal.
The vowel sound in that last syllable is the unaccented vowel we call a "schwa" that occurs everywhere in English. It's the same last-syllable sound as in "Marian" and "Sheraton."