Can a MOSFET be used for dimming instead of switching a LED?
It is not recommended (especially for 3 A) to turn an LED off and on for dimming purposes. You should use a constant current LED driver that delivers the current in continuous conduction mode (CCM).  CCM means that the LED current never goes to zero between switching cycles.  An inductor is required.  

  











This is a simple Buck CC LED driver circuit using a 3 Amp Didoes Inc. AL8849. 
Max current is set using RSET.
You would use the CTRL pin for PWM dimming







The easiest and arguably the best way to do this would be to use an AC powered $45 Mean Well 42V, 120 Watt HLG-120H-42B Constant Current Driver with three dimming methods utilizing a single pair of wires.  This driver provides 93% efficiency from wall to LED. 

This is a high quality driver with a 7 year warranty. There are cheaper knock off drivers available as well.  I recommended the 42V where the constant current range is 12V-42V and max current is 2.9A.  The 36V HLG-120H-36B is 18V-36V up to 3.4 A.

DIMMING METHODS


0-100K Resistance
0-10VDC
10V PWM


Dimming methods 2 or 3 would work in your application.