what is wrong with my code

Semi colons at the end of each 'if' statements are causing your first problem, remove all three. The second problem is that all the return statements are in if blocks. This is illegal as it presents a possible case where no return statement is reached. In your code you sufficiently cover all possible numbers, but the computer does not know that and it is important to remember that you just can not do it that way. You have two options to fix this, change all your if statements to be if - else if - else statements instead; or at the bottom of that method just return one of the values (a, b, or c): options 1:

if ((a<=b) && (a<=c))
   return a;
else if((b<=a) && (b<=c))
   return b;
else
   return c;

Option 2:

if ((a<=b) && (a<=c))
   return a;
if ((b<=a) && (b<=c))
   return b;
if ((c<=a) && (c<=b))
   return c;

return a;