Specifics on testing gravity

This Saturday will be exactly 3 weeks in the primary for my first brew, Irish stout extract,which I hope to bottle Saturday, and I wanted to open the bucket for the first time and take a gravity reading to make sure it's ready

My question is- I've read several times that you should take tests 3 days apart to make sure. Now should test one be today since Saturday is 3 days away or should Saturday be test 1 and Tuesday bottle day(if fg is same)?

After three weeks more than likely fermentation is finished. That recommendation is really for the noobs who think they need to rush their beer off the yeast cake into secondary, because of some outdated thing they read. Since you already WAITED (Though personally I would wait another week) then you gave the beer enough time to finish, so you really only need one grav reading on bottling day to determine the abv of the beer, AND if you plan to carb per style.

It shouldn't make any difference. There won't be any significant changes between 21 days and 24 days in primary, unless you have a serious problem (contamination or stalled fermentation).

Anything that says the beer will be ready in X weeks is just a suggestion. The hydrometer and your taste buds will let you know when it's ready.