Thank for your help everybody.
Ok, I think this last solution is to avoid because while my form is submitted, the form automatically displays the next step (too quick, so I replace this by a smart campaign in Marketo) or I have to find a command to hide the display faster.
Last question but not least:
Let's pretend somebody has reached the last step and the landing page displays the form not in a lightbox form(not pop-up) and so no additional is required; however there is still the button:
""click here to update my info" => which show all fields from steps 1, 2, 3 with pre-filled values"
So I end up with a form which might contains a lot of fields, is there a way to swith the display of the form by a lightbox/pop-up one if somebody clicks on the button and if he close it without filling it the form is replaced by a button to re-open it please?
Meant to include link to examples: http://developers.marketo.com/rest-api/assets/forms/examples/