You would be surprised[0]. In low level code (in some cases) you'll need memory barriers[1] to ensure the proper order of instructions execution.
Even in higher level code your compiler's optimization algorithm may decide that it's faster to execute step 3 first if it thinks the result would be the same.
[0] https://en.wikipedia.org/wiki/Out-of-order_execution
[1] http://irl.cs.ucla.edu/~yingdi/web/paperreading/whymb.2010.0...
You get into trouble when you:
1) assuming ordering to external peripherals (e.g., memory mapped I/O)
2) assuming stores/loads are happening in a specific order (e.g., other threads either pre-empt or are simultaneously running) causing data races
Your response makes it sound like a compiler could reorder functions like:
x=5;
x=add(x,5);
x=multiply(x,10);
printf("%d", x);
Take this example:
a = 5;
b = sqrt(44);
return a * b;
In your example changing lines 2 and 3 will alter the result naturally so the optimizer won't touch it.