Not a surprise...
First, not a surprise that a brand new unit, that just came out in Japan, that is not being advertised anywhere else (I've never heard of PS Vita until now...), and has how many games that are written specificially for it?.... is selling fewer units than the established system.
Second... "three times fewer" is a negative number. If the PSP sold x units, x-(3*x)=-2x. I know people are now abusing the English language by saying "3 times fewer" when they mean 1/3rd as many, or "3 times more" when they mean 3 times as many, but it's wrong and I request you don't do it please 8-) And, no, it's not just pedantic, ads here in the US are now saying things like "3 times more" when they really mean "3 times as many", inflating the figures on how sweet their soap or credit card or whatever is.