I was thinking to replace them with three LL120s but cause I'm incredibly new to building but am still learning. I'm also incredibly aware that this question may have been asked before. :)
LL120s are really bad radiator fans. They're just bad fans for performance in general. Yes, they look nice, but you're temps will get hotter if you replace the Aer P fans that come with the X72. For radiator fans, you want good static pressure ratings, so at the very least, check the specs for the fans for yourself. Otherwise, you're free to do what you like with your system.
Would the AER NZXT Fans yield better results, if not similar?
Do you mean the Aer RGB fans? NZXT has Aer P which are radiator fans, and Aer F which are standard case fans. I think the Aer RGB fans are the same as the Aer F fans just with RGB. As I mentioned before, you should probably check the fan specs and compare their ratings. But the X72 Kraken already comes with a set of 3 Aer P fans (I have it and use them with an overclocked 9900k @ 5.3GHz).
To give you an idea, radiator fans have a higher static pressure rating and generally higher RPM than regular case fans. For regular case fans, you are looking for a higher CFM. And for all case fans, lower dBA is better as most people prefer quiet fans.
can you use the AER RGB fans with radiators or is it better to just use the stock fans?
The Aer2 RGB fans have a very low static pressure rating and wouldn't make good radiator fans. For the best results, either use the Aer F fans that came with your AIO or replace them with fans that have a comparable or higher static pressure rating: Aer F's are rated at 2.93 mmH2O, while the RGB2's are only rated at 1.35 mm H2O.
Some good radiator fans: Noctua NF-F12 PWM (2.6 mmH2O), Corsair SP120 (3.1 mmH2O), Corsair ML120 Pro (4.2 mmH2O). Noctua NF-F12 IndustrialPPC 2000 RPM version (3.94 mmH2O) or 3000 RPM version (7.63 mm H2O). Just keep in mind that the higher RPM fans like the SP120 and especially the Noctua 3000 RPM are going to get noisy.
As mentioned above, use the fans that come with the X72 Kraken. They bundle the correct fans together with the cooler so that you don't have to go out and buy anything else.
Also, the user above probably means Aer P, not Aer F. The X72 Kraken comes with Aer P which are NZXT's radiator fans.
They also mention Corsair ML120 fans as good radiator fans. If you want Corsair fans, you may as well just get the Corsair H150i Pro as they already come with a full set of ML120 fans. Just about all 360mm coolers are pretty equal when it comes to performance, and they all come with the correct fans that you should be using with them. Things only start to get complicated and bad if you switch fans around, unless you know what you are doing. If you don't know what to look for, don't change the fans. Just use the fans that come with the cooler.
Yeah, I meant the Aer P's, sorry.