Deven - Shashi * Hemant means Shashi is younger than Deven, and Shashi and Hemant are of the same age. Thus, Deven is the oldest. Now, the opposite statement would mean : Deven is the youngest.

(i). Hemant + Shashi + Deven means Hemant is younger than Shashi, who is younger then Deven. So, Deven is the oldest.
(ii). Hemant - Shashi + Deven means Shashi is younger than both Hemant and Deven. Thus, either Hemant or Deven is the oldest, but Deven is not the youngest.
(iii). Shashi * Hemant + Deven means Shashi and Hemant are of the same age and Hemant is younger than Deven. So, Deven is the oldest.