So Taunton will lose the current fast service to London and end up with just a semi-fast service calling at several stations on the Berks and Hants and extending the fastest journey times from Taunton to London by about 20 minutes? If you are running three high speed services per hour from London to Exeter then I'd have thought at least one of them could stop at Taunton. You could possibly remove Newton Abbot from the from one of the two Plymouth services per hour to create a "super fast" service which could then call only at Reading, Exeter, and Plymouth. The second service could then call at Reading, Bristol, Taunton, Exeter, Newton Abbot and Plymouth. I'm not sure there would actually be demand for two high speed services per hour to Plymouth though.
Ok, so slight amendment:
1tph - London, Reading, Exeter and then onto Cornwall calling at most stations (including Plymouth)
1tph - London, Exeter, Plymouth
1tph - London, Reading, Bristol, Taunton, Exeter, Newton Abbot, Plymouth
2tph - London, Oxford fast
2tph - London, Reading, Oxford
1tph - London, Reading, Swindon, Gloucester, Cheltenham
2tph - London, Bristol
2tph - London, Reading, Bristol Parkway, Newport, Cardiff
However even with an extra 2 or 3 stops on the B&H line the journey time may not be that much slower than at present as by the time of HS3 the B&H should have been electrified meaning that accerlartion is better.
I'm also not sure even Exeter would have demand for 5 tph to London(3 high speed plus 1 semi fast via the Berks and Hants and 1 Waterloo).
Firstly, there could be more demand in 2040 than at present.
Secondally the service to Waterloo and the service via B&H would likely only attract passegers which were going to places along those lines and not into London (e,g, Westbury, Newbury, Salisbury, Basingstoke, etc.), in the same way that you wouldn't argue that the Waterloo services should be cut if there are more services on the B&H.
Finally, it could well be that by the time this HS line is built that there could well be 2 services to Waterloo (plus 1 on the current service pattern) and 3 trains an hour along the B&H (plus 1 on the current service pattern), meaning that even with 3tph on the HS line, 2tph going to Waterloo (3 to 5 coaches), one semi fast via B&H (3 to 5 coaches rather than 9) and one slow via B&H (3 to 5 coaches rather than 9) that the increase in (viable) services to London wouldn't really be that much more than at present