In theory you can do something like this:
julia> a = randn(4, 4)
4×4 Array{Float64,2}:
-1.93222 -0.427925 -0.502227 0.522741
0.240474 -1.13067 1.68355 0.839821
1.79882 0.918598 1.28506 -1.38715
0.0540847 0.197535 -0.204077 -0.282407
julia> Base.show(io::IO, f::Float64) = @printf io "%1.3f" f
julia> a
4×4 Array{Float64,2}:
-1.932 -0.428 -0.502 0.523
0.240 -1.131 1.684 0.840
1.799 0.919 1.285 -1.387
0.054 0.198 -0.204 -0.282
but I don’t know whether it’s recommended, or indeed how to undo it… 