What you have is a vector, not an array. You can use rollapply
function from zoo package to get what you need.
> x <- c(1, 2, 3, 10, 20, 30)
> #library(zoo)
> rollapply(x, 3, sum)
[1] 6 15 33 60
Take a look at ?rollapply
for further details on what rollapply
does and how to use it.