Maybe:
julia> Iterators.product(a,b,c) |> collect |> vec
27-element Vector{Tuple{Int64, Int64, Int64}}:
(1, 4, 7)
(2, 4, 7)
(3, 4, 7)
(1, 5, 7)
(2, 5, 7)
(3, 5, 7)
...
Maybe:
julia> Iterators.product(a,b,c) |> collect |> vec
27-element Vector{Tuple{Int64, Int64, Int64}}:
(1, 4, 7)
(2, 4, 7)
(3, 4, 7)
(1, 5, 7)
(2, 5, 7)
(3, 5, 7)
...