These three achievements, Grey Warden, Master Warden, and Blight-Queller, are gotten from killing Darkspawn, respectively 100, 500 and 1000. This can theoretically be done in one playthrough, but require some luck and finding random encounters when travelling.
Luckily, the key here is that the achievement description should have an added "Across all playthroughs", as the count will tally up how many you have killed on all your characters. Saving, killing darkspawn, and then reloading to kill the same again will not count.
To see how far along you are, with the back button, find the leftmost book icon and then scroll right to Heroic Accomplishments. On the Party Accomplishments page, look at kills and Darkspawn killed, on all your characters, for the grand total.
If you are close to 1000 but have finnished the game, the easiest would probably be to start a new Dwarf Noble story for some quick access to more Darkspawn, or play through to Ostagar and then do the Korcari Wilds and Tower.