Maintaining Constant Input Voltage (DC) and Limiting Power Drawn from a Power Supply
If the power supply is a conventional one with a regulated output then there should be a 24VDC output with loads ranging from zero all the way up to 3A. There may be some variation within the specifications for such regulated unit such as for example 24VDC +/- 5%.

There are some types of regulated power supplies that may require a minimum load in order to maintain the regulated output at 24V. Usually this minimum load requirement would be specified in the power supply data sheet. For example it may be specified as a percent of full load (FL). In such case if specified as 10% of FL then you would have to arrange the load to be at least 0.3A to see the 24VDC output within voltage range.

There are also power supplies that have unregulated outputs. On these the output voltage will vary based upon how much load current the power supply will provide. Generally it is the case that at lower load current the output voltage will be higher and as load current increases the output voltage would decrease. The relationship between voltage and load current may not necessarily be linear. If you have this type of power supply and yet your load requires a fixed 24VDC then you have two things to consider. (1) Replace your power supply with the regulated output type as described above. Or (2) add a voltage regulator between the supply and your load. This latter solution does require that the supply output be a high enough voltage so that the regulator operates within its specification to produce the needed 24VDC output at the required load current. The unregulated supply minimum input voltage to the 24VDC output voltage difference needed will depend upon the type of voltage regulator used (linear or switching) but may need to be 3 to 4 volts.

Finally let us consider the subject of how much current the power supply gives to your load. If the load requires a certain amount of current at 24VDC then the load will draw that much current. It will not be forced to take any more. It is a common misconception for people new to electronics to think that a load has to swallow up the full rated capacity of the power supply but this is not the case.