When the first two new sets of Mk5's arrive, they will go in on the Scarborough service, as the Mk3's would've done. This enables TPE to have a longer turnaround at the station (73 rather than 13) and ensures services are no longer turned back at Malton, unless there is a major issue like on Friday. There's now going to be longer turnarounds at the Airport too, also enabling services not be be terminated at Victoria or Pic - although PPM will still struggle given the Oxford Road corridor.
Plus even if the Mk3's were in use, all it would mean is that TPE wouldn't be loaning a 170 from Northern. Oh and there's at least one 350 out of action following an animal strike on the WCML, which is why there's been 185's running on the route, including Friday.
So, no. Mk3's would not be a simple capacity solution. And the Mk5's will be out testing before Xmas - to get back on topic.