Of the three up services which were previously 2x317, from experience:
1) The first of the three was fairly empty at Hitchin (especially towards the rear), took on a small number at Hitchin, more at Stevenage, but filled heavily at Knebworth and Welwyn North, but still arriving at Welwyn GC with a fair few seats available.
2) The second of the three was modestly located arriving at Hitchin probably with at least one person in every bay of four/six seats. By the time of arrival at Welwyn GC most seats would be taken.
3) The third of the three was somewhere in between the two.
All three would be emptier on Fridays or during school holidays.
Does that tie in with your own experience?
This is consistent with my experience at WGC. I am guessing they end up several groups of passengers:
1) Passengers from the 'village' stations on the Cambridge Branch who don't then interchange for faster services at for example Royston.
2) Passengers from the Cambridge branch travelling to Finsbury Park rather than King's Cross.
3) Passengers from Knebworth, Welwyn North and WGC travelling to Finsbury Park or King's Coss, where these are the fast trains (aside from the odd faster service at Knebworth and Welwyn North that are non-stop to King's Cross).
4) Passengers travelling from the Cambridge branch whose destination is employment in Stevenage or WGC (or Hatfield interchanging at WGC in the up direction.
5) Lastly, passengers not in the above groups who use these trains to get a seat rather than stand on a faster service.
Thameslink will provide more and longer trains from stations on the Cambridge branch stopping at Finsbury Park, so the number of passengers in group 2 using these trains should reduce once the Thameslink timetable commences. Against that Hatfield and Potters Bar stops will be added according to the Timetable Consultation document, although whether there will be alterations in the draft timetable remains to be seen...
Of course on the corresponding northbound PM Peak trains you can add passengers who saw Cambridge on the concourse information screen and realize that their journey will be 45 minutes longer than they hoped as Sonia finishes listing the calling points in Gas Works Tunnel!