Well yes, you can know.
First three important facts that affect what you get.
1. All "normal" fan headers on mobos can supply up to 1.0 A max to the total load connected to the header. So IF you are using fans that each use 0.3 A, you CAN afford to connect three of those to one header using a Splitter, but not four.
2. Every fan generates a fan speed signal which consists of 2 pulses per revolution, and is sent back to the mobo header on Pin #3 of the header. But the mobo pulse counting circuitry can NOT deal with multiple pulse streams coming in from more than one fan. If it were given such a mixed-up and changing signal mess, it would give wildly changing results and speeds readings ranging from near-zero to impossibly high numbers. That would be meaningless and, worse, might trigger the failure monitoring detection systems to signal fan failures and shut the system down. SO, ALL decent fan Splitters and Hubs will send back to their host mobo header only ONE of their fan's signals, and ignore all the others. They have ONE output male connector that is capable of this, and all their other male output connectors skip the Pin #3 signal.
3. A mobo header does NOT need or use the fan speed signals for control of fan speed. Automatic Speed control is based entirely on temperature measurements. The mobo CAN show you the speed of some fans for your information, and DOES use that info for another purpose: failure detection. It monitors the speed signal of fans on its headers and, if it detects no speed (or in some cases, speed below some minimum setting) it sends out an alarm and MAY take action to protect itself from overheating. So, when you use Splitters or Hubs to connect more than one fan to a header, not all of them can be monitored for failure. That leaves it up to you to check from time to time that all fans are working.
IF you are using a Splitter or a Hub to connect more than one fan to the CPU_FAN header, it is important that the fan that actually does cool the CPU is the one connected so its speed signal is sent to the mobo. All mobos do extra monitoring of the speed signal on the CPU_FAN header and send out warnings if they detect failure of this important fan, often taking further action to shut down quickly even before waiting for the CPU internal temperature to rise.
Splitters and Hubs are two different types of device for connecting more than one fan to a single header, but too often they are mis-labeled, in my opinion. Here is how to tell them apart.
A SPLITTER has a single input arm ending in a female (with holes) connector that plugs into a mobo header. Then it has two or more output arms, each ending in a male (with pins) connector to which you attach a fan. It has NO other types or arms. It gets power from all of its fans solely from the mobo header, which normally is limited to a max of 1.0 A current. It extends to all its fans whatever control process the mobo header is using.
A HUB may appear to be a collection of arms of wires, or a bare printed circuit board with headers, or a box with cables from it and ports for fans to plug in. It will have a single arm with a female connector to plug into a mobo header, several output connectors or ports for fans, PLUS a third type of cable arm. This third type MUST plug into a power output connector (either SATA type or 4-pin Molex) directly from the PSU. It uses this third arm to provide all the power for its fans, thus avoiding the limit of the single mobo header. A Hub MUST have a PWM signal from the host mobo header that it can forward to its fans for controlling their speed. In almost all such cases, the hub simply shares that signal out to its fans, and thus the fans used with this system MUST be 4-pin PWM fans that can use that signal to control themselves. So for the most part, a HUB can be used only with a mobo 4-pin header that actually is using PWM Mode for control of fan speed, and can be used only with 4-pin fans.
So, a SPLITTER can power and control either 3-pin or 4-pin fans, but it is limited to the header's 1.0 A max current to all fans it supplies from that single port. Of course, if used with 4-pin fans, it must be a 4-pin Splitter. But a 4-pin Splitter CAN be used with 3-pin fans as long as it is plugged into a header that uses the older Voltage Control Mode. (A 3-pin fan connected to a header using the newer PWM Mode for control will always run full speed.)
A HUB avoids the current limit of a mobo header and thus allows more fans to connected to the single header and controlled by the same signals, but it must be used in a fully 4-pin fan system.
In both cases, on each header used with one of these devices, only ONE fans' speed signal will be returned to the header; the speeds of the others on that Splitter or Hub will be ignored.
OP, your plan using fans that consume 0.3A each is to combine three on one mobo SYS_FAN header, and two more on the CPU_FAN header. That can be done easily with simple SPLITTERS - you don't need any Hub to do that. From the perspective of max amp load, that is quite all right.
Your plan has one small consequence that you MAY want to reconsider, maybe not. The fans on the CPU_FAN header are controlled automatically by the temperature measured by a sensor inside the CPU chip itself. The fans on the SYS_FAN header are controlled by a different sensor built into the mobo, so that is ideal for case ventilation. Your plan would put both fans on the CPU_FAN header (presumably, one to cool the CPU and one to help with case ventilation) under control of the internal CPU temp sensor. IF you wanted that case fan to be under control of the mobo sensor (just like the other three fans), it might be possible. That depends on details of the fan type you use and the capabilities of the mobo SYS_FAN header, and would require a Hub. IF you want to look into that, post back here exact model numbers for your fans and your mobo. Otherwise, just go ahead with using two Splitters as you plan.
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