If I am running the 3 Night's Watch North Agendas- characters gain Military, Intrigue and Power Icons, can I use street of steel (/street of sisters/shadowblack lane) to reduce the cost of a character who does not have a printed military icon (/power/intrigue) but gains one due to the agenda being run?

If you don't mind me saying, a quick search reveals that there's at least 4 threads over the past year that cover this question. That being said, the answer is "no." The agendas only affect cards that are in play.

Specifically, since the Agendas only work on the cards when they are in play, you cannot reduce the cost to play the character based on a characteristic that it will not have until after you have successfully played it.

Effectively, the Streets can only be used on characters that have the respective printed icons.