For a plenary you have to
- go to confession within 20 days before or after
- receive the Eucharist on the day of, or within a few days
- be in a state of grace when you do the work
- recite prayers for the intentions of the Holy Father
- be free of any attachment to sin, even venial sin
If you are not in a state of grace (or not a Catholic) you cannot earn indulgences.
If you do a plenary indulgenced work and you are in a state of grace, but you do not complete all the above conditions, then your indulgence is partial.
Likewise, not all indulgence works are plenary indulgenced works, many are only partial.
You refer to the current Manual of Indulgences to determine the list of indulgenced works and the other rules.
The 3 rules you mention were not the case for the Holy Doors a few years ago. I went through probably 2 dozen Holy Doors for indulgences that year and can confirm that the rules in place were the ones I listed above.