Is Lenggeng, or Pajam or Mantin or all three in a valley?

Yes, Mantin and Pajam sit in lower basins/valleys relative to the surrounding hills; Lenggeng is a more mixed/hilly area (some settlements are in valleys between ridges, but much of the sub-district is higher terrain).

All three towns are indeed in a valley-like setting, surrounded by the hills and mountains that form the larger topographical features of Negeri Sembilan.