There are some two word keywords in the google keyword tool that have next to no competition, are these still harder to rank for than a three word keyword with a little competition? Then even a four word keyword with moderate competition?
google kw tool is not accurate.. they just show estimate result.. just like what the title said "suggestion" tool..
Different keywords or key phrase has different levels of competitiveness, what you see in Google keyword research tool is not accurate and only estimated. Another place to analyze how competitive the keyword is.. by analyzing those sites that are already ranking, if they are well-established and have high-reputation enough to outrank..
If the competition is same for a two word keyword and a three word keyword then I think the difficulty to rank is the same also
a two word is definitely more difficult to rank for in inner pages, but if you have the keywords in your domain whether it be two or three, its not going to make that much of a difference.