The odds of being dealt three suited sevens in a six-deck game are approximately 63,000 to 1. [Source: Arnold Snyder, "The Big Book of Blackjack" p205]
The odds of being dealt three sevens (not suited) is somewhat worse than 500 to 1 (considering that casinos frequently offer the payout of 500 to 1 for players receiving three unsuited 7s in the "Super Sevens" side bet).