As part of the re-quadrification, there is a new ladder going in at Wellingborough South Jn, about half a mile south of Wellingborough station, with right-handed crossovers Up Slow to Down Slow, and Down Slow to Up Fast - thus enabling trains to depart platform 4 and gain the Up Fast, thus avoiding Sharnbrook Tunnel. That said, since there is no access from Down Fast any platform except platform 1, it
is still the case that anything calling in platform 3 northbound must have come through Sharnbrook Tunnel in the down direction.
Looking at the May 2021 timetable, the only things I can see booked in 3 are freights:
Train information at Wellingborough between 0200 and 0159 on 16/06/2021. From Realtime Trains, an independent source of train running info for Great Britain.
www.realtimetrains.co.uk
The up Corby-St Pancras services are generally booked in 4, as you say, but looking at such schedules for the new timetable, e.g.:
Realtime Trains is an independent source of live realtime running information for the Great British railway network.
www.realtimetrains.co.uk
The line code "UFL" at Wellingborough South Jn indicates the train is planned to cross Up Slow to Up Fast there, so no use of Sharnbrook Tunnel there.
However, there are a few trains that do use Sharnbrook Tunnel - the way to tell is to look for line code "DSL" or "USL" - you may have to look back as line codes are only declared (at least on the MML) where the line changes.
For example, the first train of the morning, 1H00 0432 Corby-St Pancras:
Realtime Trains is an independent source of live realtime running information for the Great British railway network.
www.realtimetrains.co.uk
declares line code "USL" at Kettering North Jn, and then no further line codes until Bedford, indicating it is booked to use the Up Slow Line through Sharnbrook Tunnel (and corroborated by the fact it's booked 19½ minutes from Wellingborough to Bedford, rather than the usual 11½, with no allowances beyond the usual [1] engineering time).
On Monday mornings, the first train from Derby, 1B01 0356 Derby-St Pancras:
Realtime Trains is an independent source of live realtime running information for the Great British railway network.
www.realtimetrains.co.uk
is again booked over the Up Slow from Kettering North Jn to Bedford North Jn.
Interestingly the schedule for 1B01 for Tues-Fri has enough pathing time in it to give the same arrival time at St Pancras:
Realtime Trains is an independent source of live realtime running information for the Great British railway network.
www.realtimetrains.co.uk
I haven't seen any others booked to use Sharnbrook Tunnel - do let us know if you spot any more!