Decent is all buttoned up. Descent has all the fun because it gets to climb down a mountain. Dissent is what you do when the glee club wants to get matching red outfits but you like purple.

Decent, pronounced "DEE-sent," means "socially acceptable." It's also an old fashioned way to say "polite." These days if you ask someone if she's decent, you probably don't care if she went to finishing school, you're just wondering if she's dressed so you can come in. Decent can also mean good but not excellent. Have it both ways:

He's a decent old chap, but isn't at home much. (Bertram Mitford)

"We're going to work hard out there, and earn a decent living." (New York Times)

Descent, pronounced "dih-SENT," means the act of moving downward, either physically or socially, like an evil cartoon character's descent into the underworld, or a villain's descent into madness, or even your descent into the subway station or down a mountain:

There are a few nasty, short climbs, but several long descents too. (Guardian)

Descent can also refer to a person's cultural background. Makes sense if you think about going down the family tree:

My mom, who is of Persian descent, was born and raised in the States. (Here to Stay)

Finally, dissent, also pronounced "dih-SENT" is to pipe up against popular opinion. It can refer to an opinion that differs from an official or popularly held one.To dissent is to voice such an opinion. There are shades of meaning, but it always means to go against the grain:

Mr. Putin, who has rarely faced public dissent in his 11 years in power, appeared momentarily taken aback, but quickly recovered and finished his statement. (New York Times)

Mrs. Twining curled her mouth in bitter dissent. (Edgar Fawcett)

To recap: decent is fine and you're dressed, descent is to go down down down, and dissent is when your opinion doesn't mesh with others.