I have a feeling that planning a full 401 connection is the next step in the process after they get this initial section past the design phase. This project can move this fast because a lot of work has been done in advance - the right-of-way for a twin roadway is already graded most of the way through and functionally just requires a handful of new overpasses and interchanges, with very little of this being on a new alignment.
A third section westward connecting to the 401 with a freeway-to-freeway interchange will arguably be more complicated and take longer since it would require both a brand new alignment and a more complicated interchange structure. Getting the basic 4-lane connection with Colonel Talbot/Sunset was likely deemed satisfactory in the meantime so they don’t have to rush the full freeway connection.
If this Highway 3 freeway ultimately connects to the 401... you have a chance to perhaps renumber it... as the 403!
The current 403 could be renumbered to something else... with the Gardiner now going into provincial hands you could maybe have 403/QEW-403/QEW/Gardiner become a new highway number. The QEW itself would just run from Burlington to Fort Erie.
You could even number it 402 all the way to DVP (possibly 404?) with a cosigned section with the 401 between London and Woodstock.
Way too early to know if the DVP/Gardiner will get numbers but its fun to spitball for sure.