Generating spatially uncorrelated random fields

I have DEM (cellsize 10x10 and cells.dim 450x300) and I need to create 100 random error grids that have the same dimensions as the DEM.
The values in the error grids are generated from a normal distribution that has mean of zero anda standard deviation equal to the RMSE of original DEM ~ (0, +-3).
Then I need to add those error grids to the original DEM and create 100 realisation of the original DEM.

Any help is appreciated. Thanks!
well...I don't have a clue how to do that in R:)
My DEM is SpatialGridDataFrame and I looked some functions (eg. GaussRF) but they all need covariance or variogram model, and I only have rms error.