From the rulebook:
Construction: "Transfer one Resource from your Stock to one of your Buildings under Construction". Just the one, for the Order. Plus one for each Construction Activity. So if you have played a Construction order, and have two construction activities, you can:
(i) start 3 new buildings;
(ii) transfer 3 cards from stock to buildings under construction;
(iii) any combination of (i) and (ii) above to a total of 3.
i would further describe (ii) as "transfer 3 cards from stock to buildings which were already under construction at the beginning of the turn."