I keep seeing this form often: div 3, 15
this is not any valid INTEL mneumonic!
To divide 15 by 3:
xor edx, edx
mov eax, 15
mov ecx, 3
div ecx
For the second error, you cannot move a 16 bit register into a 32 bit register like that. You need to use one of the following:
xor ecx, ecx
mov cx, ax
Or:
movzx ecx, ax