For example, if someone says:
Looking at the next three years, I think stock prices will drop,
then does the phrase "two-years' time" mean at the end of the next three years, in the next three years, or at some point in the next three years?
Rather than suggesting that a) the stock price will drop once over a three year period, or b) that it will less than it is now in three years time, I would take that sentence to mean that c) the stock price will gradually decrease over the course of three years.
When the word 'over' is used in relation to time, it is describing trends rather than discrete events. You can refer to definition number five in the OED, expressing this meaning as 'duration', for some nice examples.
It means simply that 3 years from now the stock price will be less than what it is now. Stock prices are known to go up and down quite a bit, but the speaker is predicting that the price will be less 3 years from now than it is today.