English doesn’t allow three of the same letters in a row. So Dagwood is wrong here. See the first panel:
But apostrophe-s is how you make a possessive, isn’t it? Yes, if the word you’re making possessive doesn’t already end in s. If it already ends in s, then all you need is the apostrophe. So it should be “boss’.” You still pronounce it “bosses.”
Okay, English has an exception to the no-three rule: onomatopoeic words. You can write hmmm and zoooom and buzzzz and noooo! and yasss? And so on.