Mischievous is built from mischief, MIS-chif, plus the ending -ous. Mischief came from Old French meschief, a misfortune, and at first mischievous meant harmful or unlucky. The playful meaning came much later.
The stress stays where it is in mischief, on MIS. The ie is just a weak vowel, and -vous rhymes with the end of nervous. That gives three syllables: MIS-chuh-vus.
The four-syllable form, mis-CHEE-vee-us, has been around for centuries, and some people even spell the word mischievious. Most dictionaries still treat it as nonstandard. Cambridge and Oxford list only the three-syllable form.