Personally, I do the calculations a little bit differently. I assume that I would keep my payments the same whichever rate I had, and figure out what the actual interest savings in each year would be. You didn't give actual numbers so I'll just assume your mortgage is $200,000 and you pay exactly $10,000 per year in principal, so your interest differential if rates stay the same for all three years is $200,000*.005 = $1000, $190,000*.005 = $950, $180,000*.005 = $900.
Now in this case, we've already had it hinted that the central bank will keep rates where they are for about a year, so that $1000 is pretty secure. That means that rates could go up half a point and you'd still be ahead. If they went up 3/4's of a point though you'd have $1000, -$475, -$450... still ahead, but only by $75 over the entire course of the mortgage. Anything 1% or higher after the first year and you'd have been better to lock in.
As it's impossible for the variable rate to go lower for technical reasons, and it is being goosed to the max to fight deflation, personally I think the probability is very high that when they do raise rates, it will very quickly jump up 2-4%, at least, perhaps higher. The question is, which year would that be? 2010? 2011? 2012? What happens if it goes up 4% the second year? You'd have $1000, -$6,650, -$6,300, for a total loss of $11,950.
All things considered, if I weren't waiting out the end of my prime - .75, I'd be tempted to lock in for 10 years right now.