(1/6)(1/6)(1/6) = 1/216 Tha'ts the total number of outcomes. Now have a look at the possible combinations, describes as [die1] [die2] [die3] 344 434 443 That means the odds are (216/3=) 1/72 chance...
You have to find out all combinations of dice that give you 7: that would be 1-1-5; 1-2-4; 1-3-3; 1-4-2; 1-5-1; 2-1-4; 2-2-3; 2-3-2; 2-4-1; 3-1-3; 3-2-2; 3-3-1; 4-1-2; 4-2-1; 5-1-1. I hope I didn't...