2 phase DC motor 2.8 A per phase
It is impossible to say without more stepper motor details.    

But let's make some quick assumptions:   


You are using a switching current regulated driver (TB6560 or the like)
You are using a frame power limit of about 20W for something like a Nema 17 stepper
You are not using microstepping


Here is a link to a general purpose range of steppers by Lin: 

 

While this is probably not your stepper motor is will show some of the understanding needed to size you 48VDC supply. 

With a maximum frame power of 18.65 W (both windings activated), this means a 48V current of about 400mA. So for 3 stepper motors you'd need 3* this current. 

But this is not the only consideration, you have specified that the current per winding is 2.8A. You can consider this to be the peak current that the switching driver will deliver (NOT the continuous DC current through the winding). For example consider that your stepper motor is stopped, with both windings at maximum current. The 48V power supply must be able to deliver 2 * 2.8A peak for very short periods of time, dependent on the switching frequency. (The DC current through your stepper winding will be much less than this peak. This is probably the most misinterpreted element in selecting drivers or stepper motors. When you set a driver to 2.8A. it is NOT delivering a continuous current of 2.8A)  

Normally you cope with this high peak current by having enough capacitance on the output of the power supply (again you have not supplied enough details).

So from the above you can see that to power 3 stepper motors from a 48V supply needs about 60W in total (rounding out) or about 1.25A. 

But the supply has to be able to provide peaks of 6 * 2.8A or about 17A for very short periods of time set by the switching current regulator.

You can go two ways from here: 


The classic route is to oversize the supply so you'll see a common choice being 10-15A supplies. These are capable of 500 - 750W and can deliver the peak instantaneous current requirement at all times
The better approach IMO is to select a supply that will provide the power you need for your frame choice and ensure you have enough capacitance on the output to cope with the peaks. So that perhaps selecting a 100 -150W conservative supply rating (2A continuous) with sufficient capacitance on the output will meet the need. (to select this you need to understand the driver frequency, max PWM width etc)