Hi, I bought the H150i AIO and want to replace the fans for rgb ones that came with my case, but they are not 4-pin pwm fans and the 3 connectors that come with the aio are 4 pin, I know that they'll still work but but will they ramp up when the aio wants them to?
The rad fans supplied with that system are 4-pin PWM fnas, so I expect that the power suply to them from the cable from the PUMP unit will send out that signal set. In that case, if you use 3-pin fans instead, they will not "ramp up". The will simply run full speed all the time. That is what happens when you plug a 3-pin fan into a header supplying PWM-type signals.
The H150i system is designed to control itself using the iCUE software utility that communicates with the pump unit via its cable connection to a mobo USB2 header. That software is designed to do all the control of CPU cooling plus take over control of the case cooling and lots of the lighting included in fans.
Now, all of those things can be done by features built into the mobo if you do NOT run iCUE. Well, power and control of lighting MAY be done IF your mobo has the correct type of lighting header, and you have not told us full details so we can't advise on that. But if you really want to change those fans and avoid using iCUE, then the PUMP unit needs to be plugged into a particular mobo header, and the RAD FANS can be powered and contro9lled by connecting all of them to the CPU_FAN header using a Splitter. To be sure about this, tell us these details so we can look up specs.
Maker and exact model of your mobo.
Maker and exact model of the rad fans you want to use.
Maker and exact model of all the case ventilation fans you have.
Another alternative you appear not to have considered. You plan to re-purpose the three 3-pin fans that came with your case because they have RGB lights of some type. So what are you buying to replace those case fans? By the way, you should be aware that many case ventilation fans are NOT suitable for use on radiators because they cannot work well against the air flow resistance of the finned tubes. But you could just leave the case fans in the case front as intended and buy new lighted fans ideally suited to the rad application, ensuring they are 4-pin models that the H150i system can use properly.