Meanwhile, Truro suffered their first loss in the National League Cup as a strong Hammers side proved too much for City.
The Tinners made a number of changes with the likes of Aiden Marsh, Billy Palfrey and Jake Taylor getting valuable minutes.
George Earthy – who spent last season on loan at Championship club Bristol City and has played four first-team games for West Ham – gave his side a 15th-minute lead.
Northern Ireland striker Callum Marshall made it 2-0 nine minutes later before Mohamadou Kante scored a third in the 82nd minute.
City remain top of their group and will progress to the knockout round, though West Ham could leapfrog them if they beat Aldershot Town in their final game.
“They were really good and had some experienced young lads out there who’ve got Football League experience,” added Yetton.
“Whereas when we’ve come up against some of the other teams in the competition they’ve been largely under-21s without that experience and I think that showed tonight with their quality.
“All credit to them – we got what we needed out of it for our boys who needed minutes.
“We would have liked a little bit of a better performance and a better result, but it was a worthwhile exercise for us.”
