To your main question: Yes, that is safe. As Zerk2012 says, almost all mobo fan headers today can supply up to 1.0 A current to the total load. The relevant max rated current is 0.15 A per fan. I've never quite understood what the "Safety Current" spec is from CoolerMaster, but I SUSPECT it's this: IF something really odd causes the fan to stall, that causes the fan motor to behave as if its resistance has gone REALLY low, and that allows a MUCH higher current to flow though its windings. (This is the impact of the motor's Impedance and back-emf in normal operation - "resistance" is a poor label for all that.) Now, for a motor that is NOT running because it has no power, and then is given power and DOES start up normally, there is a VERY short-term current surge lasting a fraction of a second as it first starts turning, followed by short (very few seconds) period of rapidly-decreasing current as the speed comes up to normal operation. That is the start-up current surge, and the motor AND the mobo header are designed to handle that brief heavy current episode with no problem. BUT if some other problem occurs that has the fan motor locked at the tine the power is first applied, OR if it is stalled while running and kept locked, neither the motor nor the power supply to it can deal with a sustained high current draw. Apparently CoolerMaster has added into the motor's internal circuitry a device that will monitor the current flow and limit it to no more than the "Safety Current" of 0.37 A if sustained high current draw occurs. This should be sufficient to protect the motor windings from permanent damage from overheating if that rare event happens.
As I said, rare. But possible. Almost impossible, however, for that to happen simultaneously for two or three fans. So, under normal operation, three fans at max 0.15 A current comes to 0.45 A max, just fine for the header. At start-up time, all three will draw a heavier BRIEF current - still no problem. IF the rare stall happens to ONE fan, then the max current draw comes to 0.30 + 0.37 = 0.67A, even if the stalled fan is not detected and the condition persists for a long time. Still not a problem.
I'll alert you to a point of confusion that I have complained about before - many makers and sellers of Splitters and Hubs use those two labels interchangeably, and they really are DIFFERENT devices! And you, OP, have been misinformed by that. To me, a SPLITTER is a simple device with one input "arm" that plugs into a mobo header, and two or more output "arms" that you can plug your fans into. It merely connects the Ground and +VDC power lines from the header to ALL of its fans in parallel so they all receive exactly the same power supply and operate exactly the same. NOTE that ALL of that power MUST come from the host header - there is no other source - so the Amp limit of the header is important. A HUB is somewhat similar but with an important difference. I has an ADDITIONAL "arm" that must plug into a SATA or 4-pin Molex output connector from the PSU. ALL of the power for the Hub's fans is drawn from the PSU directly, and NONE from the header. The Hub does also get from the header the PWM signal from Pin#4 and share that out to all its fans, but that is NOT a strain on the header. Thus the header's Amp limit does NOT apply. However, details of the electrical design mean that in nearly all HUBS, they can be used ONLY with a header that IS using the new PWM Mode to supply that required signal for sharing, AND the fans connected ALL must be of the new 4-pin PWM design to be able to use that signal for speed control. You can NOT control the speed of a 3-pin fan from a HUB. Gross APPEARANCE of these devices does not tell you which is which. Each MAY come as a collection of wire "arms", as a small printed circuit board with header pins, or as a closed-in box with ports (headers) recessed in openings. What really DOES distinguish a Splitter from a Hub is that the HUB has that extra connection to the PSU, and a Splitter does not.
OP, the Deepcool FH-04 item you linked to is a SPLITTER, which IS suitable for your purpose. Compare that to the Deepcool FH-10 unit that is a HUB with a connecting cable out to a SATA power connector from the PSU. Both look like closed boxes with port holes. The Silverstone item linked for you by Zerk2012 is a SPLITTER of the "group of cable arms" design priced at 44% of the price of the FH-04.
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