Mgmat (Page-236) is giving following examples to modify a subgroup :

(i) This model explains all known subatomic particles, some of which were only recently discovered.

(ii) This model explains all known subatomic particles, some of them only recently discovered.

(iii) This model explains all known subatomic particles, some only recently discovered.


Can somebody explain me why 'were' is used for (i) not for (ii)/(iii)