Definition from Wiktionary, the free dictionary
For me and everyone I know, the pronunciation /sə'post/ is much more common than the others, yet it's not even listed.
There's a three way distinction of pronunciation:
- /sə'post/ as in "Hey, you're not supposed to do that!" (Always followed by to).
- /sə'pozd/ as in "We supposed that he would be here because it's his birthday" (rare, usually you would use a verb like "assumed" or "expected" instead)
- /sə'po.zəd/ as in "The supposed entrance to the school" (usually found as an adverb "supposedly").