Driving strings of high power LEDs - different ways with fault protection
If you can readily get the NSI50350, I'd tend to go in that direction - or use one of the related devices.
The NSI50350 is a 350 milliampere constant current regulator.  If your LEDs are rated for 350mA and 3V and and you are using a 24V power supply, just stack 7 LEDs in series.  That'll get you 350 mA to the LEDs very easily.  7 LEDs gives you a little &quot;wiggle room&quot; on the voltage.

PWM is also easy.  Just use a low side transistor (BJT or MOSFET ,which ever you prefer) and drive it with PWM.  The NSI50250 datasheet shows how:

ON has a bunch of related chips (linked to above) that all work the same way. Put one in series between the LEDs and voltage source, done.  PWM with a transistor in series.
If you keep the voltage source close to the total voltage of the LEDs (plus headroom,) then the chip won't get very hot - but make sure you do have adequate heat sinking for the chip and the series transistor for the PWM.