I've been through this process myself and there is a twist to the whole process.
You need to have the 3 Weeks / 750 Merits combination and the next tick will give you rank three (and reduce the merits).
However, you may not have acces to the prismatic shields at once, because for them you need to be pledged four entire weeks.
That confused me a lot because I pledged on a Monday and had to wait three additional days after the tick to actually reach the "be pledeged for four weeks" condition.
You need to have the 3 Weeks / 750 Merits combination and the next tick will give you rank three (and reduce the merits).
However, you may not have acces to the prismatic shields at once, because for them you need to be pledged four entire weeks.
That confused me a lot because I pledged on a Monday and had to wait three additional days after the tick to actually reach the "be pledeged for four weeks" condition.