Does Seeds of Strength trigger Heroic three times?
Only once. The actual trigger condition is "whenever you cast a spell...". And there was only 1 instance of "cast a spell" happening.

IF the ability said "whenever a spell targets Sage of Hours..." then it would happen 3 times.

See the Gatherer ruling on Sage of Hours:


  Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.