I have the location "Pentos" in play and I want to ambush the character "Long Lances" into play.

The relavent text on Pentos' reads "Lower the cost of the first Army character you play each round by 1".

Does Pentos reduce the amount of influence required to ambush Long Lances into play from 3 down to 2 ?

Similarly, Summer Sea's text is "Discard Summer Sea from play to reduce the cost of the next Targaryen or Martell character you play this phase by 2"

If I discard Summer Sea, can I reduce the cost to ambush my next Targaryen character by 2 less influence ?

The cost reductions only apply when you "play" the card (from your hand, during marshalling, by paying its gold cost). Ambush is a "put into play" effect. "Put into play" effects ignore the restrictions that usually come with "playing" a card, but also remain unaffected by anything that affects you "playing" a card.

For an example how a card that lowers the cost of Ambush would be worded, see True Queen's Harbinger.