Be prepared to make a starter from the yeast you plan on using in 3 weeks, and depending on your OG for the one this weekend, you may want to make one for that as well. The dry yeasts will last refrigerated for months/years.
By the way, making a yeast starter does not have to be intimidating. Boil 2&3/4 cup water, add 1/2 cup DME and a pinch of yeast nutrient(if you have it). Boil for another 10 minutes or so, and let cool naturally. Sanitize a 1 qt. canning jar, either using starsan, or by boiling for 10 minutes, add the contents of the yeast packet plus the cooled starter wort. Use tinfoil to cover the top of the jar, place it someplace warm and give her a shake every time you walk by. It's ready in anywhere from 1 day (called a vitality starter) to 4 days. Can be used or refrigerated for future use.