Ok, many thanks for your help.
I thought that there has to be an easier solution, for instance double the Main Body in the Layout of Helix Ultimate, one for desktop, and one for mobile (configured with the Responsive options), but in the Layout I can define only one zone as "Component".
I have found also a solution  naming Custom CSS Class for Left "order-2 order-lg-1", and so on, with the following Custom CSS:
/ Default desktop styles /
.main-body {
display: flex;
flex-direction: row;
}
.left-area {
order: 1;
}
.component-area {
order: 2;
}
.right-area {
order: 3;
}
/ Mobile/tablet: stack vertically and reorder /
@media (max-width: 991.98px) {
.main-body {
flex-direction: column;
}
.component-area {
order: 1;
}
.left-area {
order: 2;
}
.right-area {
order: 3;
}
}
but there was missing one step...
I have "solved" now my problem, by no showing Left for mobile with the Responsive options, which is not what I initially wanted, but ...