Which part of BC are you in? If you're in western BC, it might be a little difficult to keep it dry enough. In some areas, it might be difficult to keep it cool enough in the summer.

The main thing to do to keep seed viable is to try to keep it cool and dry. You'll have to do some conversion here, because I'm going to use a rule of thumb that relies on Fahrenheit. The rule of thumb is that you want to store the seed so that the combined relative humidity and temperature (in Fahrenheit) stays below 90. For example, if it's 70 degrees and the relative humidity is more than 20%, you're losing seed. If it's 95, you're losing seed even if you're in the desert. The higher the combined number is, the more seed you're losing.

Over time, even stored properly, some seed will lose viability, but if it gets too hot or too moist you lose more.

Thanks. I'm in the Interior of BC. Not quite as damp as the Coast, and we get mid-summer temps up in the high 90s (mid-day). But it's not as dry here as the Pine-tree areas of our province, either.

I could keep the seed enclosed in some kind of plastic bag or container, and keep it in my basement. Temps in there probably tend to hover around 70 during the summer.

Do you think I'd get three years' viablity out of seed I bought within the next couple weeks?

If the basement stays fairly dry or if you can keep the seeds dry enough in a sealed container, you should be able to get three years out of those seeds.

Note that if it gets that hot in the summer, you'll probably want to wait until after the heat of the summer has passed to plant. If you plant now, you'll have a tough time keeping the grass alive through the summer. If you plant after the summer heat breaks, the grass will sprout late summer/early fall, overwinter, then grow all spring before summer hits, so it will have plenty of time to mature and grow a good root system before next summer.