Below is a calculation procedure I believe is accurate for solving your problem, but it cannot be presented as the work of a certified registered engineer for commercial application purposes.
Before using any of the below it is important to establish if the 48.3L/ min volume fow is the actual flow rate at 1.4 Bar or is it at some standardized Pressure and Temperature value. If it is at the 1.4 Bar pressure then the below should work. Otherwise, then this flow value should be converted to its value at 1.4 Bar before performing the below calculations. The reason I raise this issue is that here in the USA when doing any flow sizing of components we convert all pressure flows to a standardized scfm value at 14.7 psia and 60 degrees F to eliminate the pressure/temperature factor in sizing components.
What you need to do is determine the air volumes in your tank at the maximum 3 bar and minimum 1.5 bar pressures. Assuming the air temperature is constant at both pressures, The volume in your metric units at each pressure is Vmin = 20L x (1.5+1) / 1 and Vmax = 20L x (3+1) / 1. Then subtract Vmin from Vmax. This will tell you the air volume you will have to deliver and the flow rate required to refill the tank on each refill cycle in 1 minute (60 sec). Next, add the 48.3 L/min outflow rate to this value and you will have the total required inflow. I recommend you use the average pressure (3 + 1.5) / 2 as the reference delivery pressure at that flow rate. If the 48.3 L/min air volume is at the regulator discharge pressure of 1.4 Bar then a tank supply inflow volume rate at the average tank pressure should actually deliver a bit more air volume than required for filling in 60 sec but it should assure you can fill the tank within a maximum of 60 sec. However, if the outflow rate is at 1.4 Bar and you need a more accurate rate then reduce this component of required the inflow rate to 48.3 * (1.4+1) / (Pav +1) before adding it to the tank inflow volume.
If you have any questions or concerns please let know before using this calculation.