I am assuming each year has three entries and you want to fill them in with the average of the corresponding entries within those years. If that is the case:
for(i in 4:n-3){
if(list[i] = NULL){
list[i] = (list[i-3]+list[i+3])/2
}
}
I tend to be good at abstracting these, so let me know if you need help.
Whatever approximations you use the results must be indicated as "estimated". Hiding this information can compromise later the credibility of excellent conclusions you might come through.