5/3/1 is great for
certain applications. I've been using 5/3/1
inspired programs for over 7 years and have seen great gains from them. The cycles in 5/3/1 are generally too low-volume for what I like to do, even including higher-volume accessory and assistance work.
IMO 5/3/1 is made for "rugular" gym folks and not gym rats; it was originally designed to take the guesswork out of going to the gym.
Will it make you the strongest powerlifter or best Olympic lifter? No.
Will it keep you making steady progress on the Big 4 and keep you fit if you follow the program to include the conditioning? Yes.
If you want to progress as fast as possible, look somewhere else. If your goal is purely hypertrophy/size/aesthetics, look somewhere else. If your goal is general strength and "fitness" and includes something that can be done indefinitely, well there you go.
No aggy, but it sounds like you haven't read the book(s) on 5/3/1 by Mr. Big Jim Wendler.
There is no "3 week program" for current iterations of 5/3/1, there are only different cycles meant to be ran continuously. If you found this 3 week program as a standalone online, you're prolly 5/3/1ing wrong.
There is no benefit to only doing a program for three weeks and then moving on. You won't have any appreciable muscle mass gained in that time, and the strength gains or losses will be minimal and impermanent. Either stick with it for a longer haul or switch to something that's not meant to be ran like that.