Transformer are rated at their winding current. This is 24vac @ 3 amps.
Now to your original question, you are cutting the voltage tight. Quite often the transformer is 25.2 vac C.T.. If so it will help.
Let's assume it is only 24.0 vac. With full wave, grounded center tap, arrangement you will have 12 vac sinewave on either end to ground. This will give you close to a peak voltage of 1.414 x 12v = 16.96 volts peak going into each diode. Depending on your selection of rectifier you could have 0.7v to 1.4 vdc drop across rectifiier. Better if you use a Schottky diode as it will give you less voltage drop, at 6 amps forward current in order of less then 1.0 vdc. So you now have a peak DC voltage of 16.96 -1.0 vdc = 15.96 vdc.
Without a filter cap, the r.m.s DC voltage will be 11.3 vdc @ 6 amp load. If you want to raise that up a bit you can add an electrolytic filter cap. You would need about 12,000 uF to bring the r.m.s. up to about 14vdc @ 6 amps. For less current, the r.m.s. voltage will raise toward the 16 vdc peak as there will not be as much voltage slump with the large cap.
Be a little careful on lighter loads as the transformer voltage will be higher and the drop across rectifier will be less. You could end up with 18 vdc with no load.