So I want to get a fan hub for my 3 pin fans. Something big like this or this. They don’t specify whether I can plug these into a motherboard with only 1 3 pin fan socket/header. Do you know if it will work?
Neither of those will allow you to control the SPEED of the fans. The small board only can supply full power (hence full speed) to any fan. The Hub box from Arctic can control the speed only for 4-pin fans and NEEDS a 4-pin mobo fan header to do that job. You need a different approach.
For THREE-pin fans the only way to control their speed is by varying the VOLTAGE supplied to them from the mobo header. Any mobo 3-pin header will do this. Almost all newer mobos use 4-pin headers, but have options in BIOS Setup to have them behave like 3-pin headers IF you are using 3-pin fans. In both cases, though, you generally can NOT use a HUB. You CAN use a SPLITTER. A Splitter is different because it has one input to connect to a mobo fan header and several outputs for plugging in fans. But it has NO other connection, and ALL power for the fans must come from that header. A HUB (like the one you showed from Arctic) has a third connection "arm" that must plug into a SATA power output from the PSU, and that is where it gets fan power. BUT that can only work for 4-pin PWM-style fans and MUST have a PWM control signal supplied from a 4-pin mobo header.
Almost any mobo fan header can supply up to 1.0 A max current to the total fan load connected to it, so when you use a Splitter to connect two or more fans to a single header, you must obey this limit. Look closely at your 3-pin fans' labels, OR go to the maker's website for specs. There should be an Amp rating shown. IF there is none but it shows a WATTS rating, Amps = Watts/12. Add up these Amp numbers for all the fans you want to connect to ONE mobo header using a Splitter. Typically a modern fan will consume 0.10 to 0.25 A max, so it is safe to connect 3 or 4 of these to ONE header using a Splitter. IF you have more fans than that, you can connect more to a second mobo header in the same way.
Note that you CAN use either a 3-pin or a 4-pin SPLITTER for 3-pin fans. Splitters may come in three different appearances but they do the same as long as they do NOT have a cable to connect to the PSU. One form is a group of cables like this
It has one header labelled "CPU" but do NOT plug your CPU cooler into this. Use this Splitter with a SYS_FAN or CHA_FAN mobo header for CASE ventilation fans., and plug one of those into that "CPU" header on the Splitter.
It has 4 ports, but you can find others with more.
If you have difficulty figuring out the amps ratings of fans and how the limit applies, post back here with:
(a) fan maker and model, and how many?
(b) mobo maker and model so we can check its features.
With that info we can giver more focused advice.