q2: the largest sum of the numbers is the sum of the two largest numbers (in 5,6,7 the largest sum is 6+7 because 6 and 7 are the two largest numbers)
the sum of all numbers is the sum of the two largest ones plus the lowest one ( 5+6+7 = (6+7) + 5 ), thus the sum of two largest numbers is the sum of all of them minus the lowest one ( 6+7 = (5+6+7) - 5 ).
now you only have to find the lowest number. since there are only 3 numbers, algorithm lowest = min( c, min(a,b) ) is fine.