The initial proposal was Pirate Cove and Glacier Bay. Both got rejected by the HK government and so Disney came up with the three lands instead. (see the attached video below).
While TSL appears "weak" to some overseas Disney fans, it still has its reasons to stand in HK Disneyland. HKDL lacks both quantity and quality in terms of attractions, so while the rides in TSL are just (at most) "C-tickets", they can still boost the number by 3! Also, HKDL's major local rival Ocean Park is famous for its thrill rides, two of TSL attractions (namely RC Racer and Toy Soldier Parachute Drop) allows HKDL to catch up a bit in this aspect. Not to mention TS is a famous franchise in the city.