Does Heartening Soul's +3 might effect trigger when I next play a 7 cost unit and exhaust the nasus legend to ready 2 runes?

No — but the real problem is that I can't find any card called Heartening Soul. No card, errata entry, or FAQ by that name exists in my card data, so there's no such +3 Might ability for me to rule on.

Worth separating the two things in your question, though. Nasus's legend (Curator of the Sands) reads "When you play a unit, gear, or activated ability with Energy cost [7] or more, you may exhaust me to ready up to 2 runes" — exhausting him is a cost paid when that trigger finalizes, and it has nothing to do with granting Might to anything. Playing a 7-cost unit does not itself kick off any generic "+3 might" effect; only a card that explicitly says so would do that.

If you meant a card that reads something like "when you play a unit with Energy cost [7] or more, give it +3 might this turn," that trigger would fire at the same time as Nasus's and its controller would order them on the chain — but I can't confirm that without the printed text. If it's a brand-new set card, its text isn't in my data yet.