Hi Aanie
What you are going through is very similar to my experience. Indeed your Husbands AVM is almost identical in size to mine. Although mine was on the right parietal lobe close to the motor strip but the same size 3cm, an ideal size for Gamma knife treatment I was told by the consultant at Sheffield.
It may help you to read my post Gamma Knife Surgery Sheffield which will give you an insight into what to expect and help you realise that you are not alone in your thoughts and fears.
Everybody is different but I did not have much in the way of side effects but the anxiety and fear of the unknown was much worse.
I was 70 when I had my treatment and it was said that it could take 4 years to show any results. My last scan at 5 years, post op. It has shown that there is no residual evidence of the AVM ie it is obliterated. 
I hope this helps you both to see that what you are going through is not the end of the story but maybe the pathway to a life without the worry of what may or may not happen to your husband. Either way it an entirely personal choice that only your Husband can make.
Stay Positive