To answer the original question, use
vec = [MVector(0.,0.,0.) for _ in 1:4]
Then vec[1] .= [1,2,3] produces
julia> vec
4-element Vector{MVector{3, Float64}}:
[1.0, 2.0, 3.0]
[0.0, 0.0, 0.0]
[0.0, 0.0, 0.0]
[0.0, 0.0, 0.0]
To answer the original question, use
vec = [MVector(0.,0.,0.) for _ in 1:4]
Then vec[1] .= [1,2,3] produces
julia> vec
4-element Vector{MVector{3, Float64}}:
[1.0, 2.0, 3.0]
[0.0, 0.0, 0.0]
[0.0, 0.0, 0.0]
[0.0, 0.0, 0.0]