I have an overstay with a VWP of 333 days, I had to leave the US in January 2018 but stayed until December 2018 to take care of an American citizen who has no family. I have a 3 year ban. So I had to apply for a B2 visa, I have already done this twice and both have been refused. The woman at the embassy told me that I cannot get a visa as long as the 3-year ban lasts, only after those 3 years I can apply for a visa and a waiver. Is this true? Can I ask a waiver before I apply for a visa or do i have to wait until i have the B2? And which waiver should I ask?
You have zero chance of getting into the US legally while the ban lasts. That's what a ban is.
You can apply for a visa after the ban has expired, but it's going to be difficult. Since you already overstayed once the US will apply a lot of scrutiny and skepticism to your application. Your best shot is to consult with a capable immigration lawyer and create a strategy that's specific to your case and situation.
Forget about a waiver, you are not getting one. Serve out your ban, and then hope they overlook your past transgression when you apply for the visa. Going forward your chances of ever entering the USA on a non-immigrant visa are slim even after your ban is over.
Regarding the waiver, there is no form for this process. You would apply for a non-immigrant visa, and if denied solely due to a ban (and not due to immigrant intent), the visa officer may or may not recommend you for a non-immigrant waiver, which is adjudicated through the visa officer and consulate. In your case you did apply and clearly the visa officer did not feel like recommending you, so your case is dead.
A waiver is available to people who can demonstrate that if they are not granted the requested immigration benefit, such as a green card, their U.S. citizen or lawful permanent resident spouse or parents would suffer extreme hardship.
But extreme hardship is notoriously difficult to prove. It means demonstrating more than the hardship that any family member would feel upon facing separation due to denial of a visa. You’d want to be able to show severe financial, emotional, or medical consequences to one of your U.S. family members.
only after those 3 years I can apply for a visa and a waiver. Is this true?
No, this is not true. After the ban is over in 3 years, you don't need (and can't get) a waiver because you have no ban to "waive" anymore. The point of a waiver is to waive a ban while you still have a ban.
Can I ask a waiver before I apply for a visa or do i have to wait until i have the B2?
Neither. You would apply for the visa, get denied, and then may or may not be recommended for a nonimmigrant waiver if you are denied solely due to the ban, and the officer who denied you will inform you about the process if they recommend it. However, given your history it is very likely that you will also be denied for immigrant intent, which would make waivers moot, since immigrant intent cannot be waived, so waiving the ban would be useless anyway.