Have the ducks come into lay at all? being only 7mths you need to give this more time.. while i did have a duck go broody under 1yo most wait till a bit closer to that mark or beyond... as well changes in ducks routines only discourages laying.
Lights? i wouldn't bother.. ducks are seasonal layers, it is normal for them to break and also if ducks are young, often they will not lay till the following spring if they hit that average laying age in the shorter days of winter, frankly the broodiness of scovies is high, there literately is not a year that goes by that i don't thwart nests, so rarely requires encouragement other than a nice safe area to nest and good food.
Honestly, i think your trio is a bit young, i have no doubt a nest will be set but more likely in the spring.. even my ducks who always resume laying each Feb have not, been a bad winter... i had a clutch last March for comparison... but again this year no eggs as of yet and i have a wide range of ages.