Only started misfiring after the last service , no previous problems ...thank you for your help.....

I have a 2001 s type 3.0 v6, at last year's service the garage changed the 3 hidden spark plugs located beneath the manifold, they tend not to change these at yearly service because of the work involved , since then when I have driven for approx 1 hour, the engine jerks on acceleration i have had 2 ...