When numerous small decisions about related environmental issues are made (i)____, the combined consequences of those decisions often are not considered. As a result, the patterns of the environmental perturbations or their effects over large areas and long periods are (ii)____ analyzed. This is the basic issue of (iii)____ effects assessment.

The clue for blank (i) lies in "the combined consequences of those decisions often are not considered", the words "combined consequences" and "not considered" imply that the decisions are taken independently.
The clue for blank (ii) lies in "As a result", as a result of not considering the combined consequences the patterns observed over large periods of time is not sufficient, hence the patterns are not adequately analyzed.
Blank (iii) needs a word that describes something that is collective by nature. The meaning is supposed to read "This is why the collective assessment is not working", hence cumulative.