(1) is Sir John Harold Clapham, CBE, LittD, FBA (13 September 1873 – 29 March 1946), a British economic historian. Most of his works can be found on the J. H. Clapham author page; however the two pages should not be combined due to the existence of other authors called John Clapham who are not called J. H.
(2) is John Clapham, b. 1566, a historian who sometimes published under the pseudonym Philomathes.
3) John Clapham fl. 1966 Dvořák