How it works
Multiply every leg's decimal odds together, then multiply by your stake. Three legs at $2.00, $1.80 and $1.50 combine to $5.40, so $50 returns $270.
The part worth understanding
The margin is charged on every leg, so it compounds. A four-leg multi at books holding 5% each carries roughly 20% of margin against you — far worse than any single bet, which is precisely why multis are promoted so heavily.
The chance figure above assumes the legs are independent. Legs from the same match rarely are, and correlation can make the real chance meaningfully different from the number shown.