Do I have enough left in the canister?

You have a canister with about 200g of fuel .
(empty 220g canister =150g. 200g of fuel plus 150g canister =350g 12.25 oz +/- )
Takes 7-10g of fuel to boil 500ml of water with something like a Gigapower stove.
All measurements are aprox.
(Jetboil quote 10g per liter for theirs)

Will that get me through three 2-cup (breakfasts) and three 1.5-cup (dinner) boils?

Yes, very easily!

Handy conversion number to memorize is 1oz = 28.35g, and that most every 100-110g fuel canister weighs 100g empty and 200-230g canisters weigh 150g empty (+/-2g).

It's also useful right after a trip to weigh the used canisters on your scale and write the weight on the bottom of the canister with a Sharpie marker.

+1 to all above

empty canister weighs 5.25 ounces, maybe figure 5.5 ounces to be safe

write the weight on canister before trip, after trip, calculate difference, divide by number of days and you'll get the correct number for you. Do it on a number of trips including colder weather to find a worst case.

I use about 0.25 ounce to boil 2 cups

You could figure an extra days worth of fuel just to be on the safe side

The Burton canisters from Fred Meyer are about $1 cheaper than others. They're iso-butane, like others, even though they don't say it, because I use them down to 25 F.

Thanks all!

Good to know about the canisters at Fred Meyer, there's one very close by.

My older red MSR "8 ounce / 227 gram" canisters have a total, full weight of 376-378 grams.

This year's Burton "8.1 ounce / 230 gram" canisters from Fred Meyers have a total, full weight, without the cap of 375-377 grams. Those were the cheapest option in my town compared to other brands at Walmart, the warehouse outlet or Sportsman's Warehouse.

So 13.25 ounces full. Therefore about 5.25 ounces empty.

When I get them home from the store, I always right their full weight on the bottom in Sharpie. If you then record their new lower weight after each trip with a notation like:

Full = 376 g
7-21-15 = 308 grams 2 people / 3 nights.

You'll more quickly get a feel for how much fuel your style of cooking uses per person-night. And be able to utilize used (lighter!) canisters on short trips.