align_image_stack not working

I have three photos of the same scene which are a focus stack. However, it seems that my lens moved slightly (zoomed in slightly) between each of the photos so that they are not exactly aligned. I have been trying to use Hugin to align them and to then output each of the aligned images so that I can manually blend them, but I have not had success in getting them aligned. I have been using the expert interface but am struggling to find the proper combination of variables/selections that will align my photos. I would be grateful for some assistance in learning how to use the program successfully. I'm hoping someone here can point me to some assistance or would even provide the guidance/assistance I need. Thanks in advance!