In Mathematics you can make expressions like a >= b >= c, but in most programming languages, you cannot. You should do
if (a >= b && b >= c)
{
// ...
}
instead.
Also, in the last condition, you have the if and the else in the wrong order:
if (c => a && a => b)
{
// ...
}
else if (c => b && b => a)
{
// ...
}
An alternative is to keep only the else, and you remove the if and the condition, as c >= b >= a is the only remaining case, so the else should cover it, no need to check whether this is happening with the if statement.
After these changes, I think you code would work fine.
Also, I'd suggest looking into various sorting algorithms. You'll make a good use of them.
https://en.wikipedia.org/wiki/Sorting_algorithm#Popular_sorting_algorithms