So, what should the new x2 be? The average of old (x2, x3, x4) or the average of old (x4, x5, x6)?
In the first case, it's basically smoothing the data. So you can use the smooth() function with a span of 3. Eventually, you want to remove the first and the last value after smoothing as they are the first and the last value of the original vectors (no mean here).
In the second case, you can use the following function if the length of x is multiple of 3:
xavg=mean(reshape(x,length(x)/3,3));
if the length is e.g. 100, I guess you only want to use the first 99 values. Then you can use
xavg=mean(reshape(x(1:3*fix(length(x)/3)),fix(length(x)/3),3));
