To run your Star LEDs at 3W , you will need a bigger heatsink and pulse them for lower duty cycle then 1A is possible.
I have hundreds of Aluminum clad circuit boards with 18x 3W LEDs per board. I use them for all kinds of custom Luminaires inside and out.
There are many versions but the full board runs off 24V or 36V with external -DC-DC regulators.
I cut them into strips when I don't need a full board and run them off multiples of 3V for series operation up to 3.6V per LED and routinely run parallel/series arrays without any need for series R's by knowing the ESR and tolerance variations of all components in the system including the power source, thus I can prevent thermal runaway. All of mine are very consistent and have an ESR of around 1 W-Ω per LED meaning 1/3Ω at 3W or 1Ω at 1 Watt. or 10Ω at 1/10 Watt per LED in series. Lower W-Ω is better quality (but never spec'd.)
I can show you how to get more bang for the buck using MOSFET switches and use PTC"s for OCP protection for certain applications that may have a surge in voltage, but I usually run without PTC's.
You can run them off 1A at 25% duty cycle for an average current of 250mA and get far better results.
You can alternate arrays of LEDs and thus achieve constant current for some advantages and also lower temperatures.
However design of PTC thermal time constant must be carefully selected and you may elect for a precision 100mΩ current limiter with variable MOSFET gate regulation. ( or an Ultra-low LDO)
Most likely your vehicle is always running at 14.2 +/-0.2 when you want the light ON at full brightness and who cares if it dims to 50% at a low idle near 13V for example. It will still be ON at 12V.
You can therefore run 4 LED's in series with a 0.5 to 1A PTC in series. The PTC will heat up faster than the LEDs which have the benefit of a heatsink. It will regulate the current at a PTC junction temp of around 85'C . I suggest parallel PTC's like 200mA to select your desired maximum LED current and thus LED junction temperature. I would suggest the same 85'C for the LED's max. Tj.
I sell LED's in my retirement now after 35yrs of bleeding edge R&D & Mfg, and a lot more details are required to perfect what you are doing.
You can use old surplus CPU heat sinks for passive cooling in high density boards with free air assistance.
BTW, I assume you always carry written authorization from your local Fire Chief to use this vehicle flasher on your POV as a volunteer fire-fighter or medic only in real emergencies.
Although I use LEDs to avoid glare and illuminate the yard , closets and garden, you want to produce flashing glare to catch attention.
I can show some examples of 12~15V arrays of 4LED and get details on your LED of choice.
Below powered by 19V 65W laptop charger, that I modified to variable 0~24V using a custom array of 6S6P with some Amber LEDs added.
My wife wanted it brighter than direct Sunlight.
Below, for example these I made using paper lanterns ( plastic coated) with 4 LEDs cut from large board into 2"x2" square,
These are running at you might call DIM at 12.0V DC regulated brick drawing less than 2A total for 20 LEDs. If I ran at full power they would be as bright as an Ambulence Light, which is what they were designed for.
4 LEDs in series Rs=0. and 5 Lamps in parallel or 4S5P array and boards running cold under/inside shade, Wired with 30 AWG magnet wire inside to 18AWG speaker wire along eaves soffit is convenient and also prevents thermal runaway with 50~100mΩ added ESR.