From the pic it looks like it's 24,000Uf and 100V so although on the large side it could be used for 30Vdc supply.

Regards system4 board it's not 3A Max power supply required, it means the drives can supply 3A to the motors. I'd also be careful about supplying it 30Vdc if thats the Max it can handle, your always better leaving a safety margin, check it out with Roy first before giving 30Vdc.!
The Min PSU Amps required will be approx 67% of the total ampage of the number of motors used IE: 4x3A motors = 12A/67%= 8A Min PSU needed. More or less Amps wont hurt anything but less and the drives/motors will under perform when Max'd out, more and your just wasting electricty and money on more expensive transformer.
With the above example a 160Va 2x20Vac transformer wired parallel would work giving 30Vdc @8A when rectified.

Working on 4 axis system with 4x3A motors giving 30Vdc @8A then approx 16000uF would be ideal, so 24000uF is a touch on the large side thou think it will just give more smoothing and not hurt anything. Thou should always test output voltage before connecting to drives.! Dont want to let the magic smoke out.!!!