I've heard that the 3-year term is a check on the unicameral legislature and parliamentary system. Essentially, the government has license to "ram through" all the laws they want to, but if it turns out to be a bunch of ridiculous nonsense that the public hates then they get kicked out relatively quickly. Contrast this with the American system, in which the executive branch, the house of reps and the senate are all decided upon independently, often leading to political gridlock.