Your question should be broken down in two parts:
How long does it take to get a starter going?
How to schedule my sourdough feeding to fit my schedule?
For 1., if you have no starter to build on, you won’t be able to get a reliable one ready within three days. Sorry. The time until a new starter establishes, stabilizes and gets usable is at minimum four to five days1 (and would still need additional yeast just to be in the safe side), ideally longer. And that’s not even taking into account that it will likely still change it’s characteristics over the change of seasons and your interactions with it. So as today is Thursday, you may want to postpone your new Sunday baking routine by a week. If on the other hand you can get your hands on a spoonful or two of an established starter (and many bakers will be happy to share what they would otherwise discard), you can go straight to 2.
Regarding part 2., If you have a stable starter, you’ll want to establish a routine that fits your needs. To calculate your sourdough requirements, you ideally start at the end - depending on your recipe, you will need an amount X at a time point T. (Plus you‘ll need some more that you can keep for the next week!)
Now, if you are using the „sourdough on the counter“ method, you’ll be probably feeding your sourdough twice a day, discarding half of the mix and adding equal parts of water and flour. (Alternative approaches use only 10% starter and feed once in 24 hours...) In any case, you can increase the amount of sourdough by simply not discarding the excess, but also feeding that instead. Depending on the chosen method, the amount of starter needed for your baking and the amount of starter you will preserve for the next week, it should be easy to calculate how many cycles you should plan for.
If you don’t want to keep your sourdough on the counter all week, you can also keep it in the fridge (without feeding inbetween) and pull it out in time to get it ready for the next baking session. Sourdough starter can get a bit “sluggish” if stored in the fridge, so you may want to do one “get it back in shape“ feeding. I am usually going for the “feed once a day and use only 10-20% of starter” method, so for your schedule, I would
- pull the starter out of the fridge on Friday, do a “wake-up feeding” overnight, do an “increase amount feeding” on Saturday and bake on Sunday, putting the leftover starter back in the fridge until the next Friday.
You may want to experiment a bit, but the principles are always the same: Determine how much you need, increase the amount during the previous feeding(s), preserve a small amount of starter for the next cycle.
A slightly different approach (and maybe easier as you can basically skip the math completely) is to separate the cultivation of your starter from your baking. That means feeding your relatively small starter on your chosen schedule and using the excess - that what you would discard when feeding - as the basis for a leaven that will ultimately go into your dough. A tablespoon or two will go a long way here. Emma Christensen from The Kitchn explains the process in this article.
1 Unfortunately there’s always some risk that a starter attempt can fail. That’s why in baking a starter is not made again for each batch, but propagated and maintained carefully.