As has already ben stated, depends on type of material and thickness.
Flame cutting discs leaves a really hard slag edge and is murder on cutters, so I will usually go the holesaw route and cut out a disk slightly larger than my finished size. So for 3" diameter, that means using a 3-1/4" Holesaw. Remeber holesaws are sized for the hole, not the disc that is left over and often considered as discard scrap so you have to account for the saw kerf.
This will leave you with a rough washer that is just over 3" dia with 1/4" hole if you used the pilot drill, no hole if you left out the pilot. Chuck up by OD and drill, bore and finish the ID.
Mount on a suitable mandrel and turn the outside to dimension. Now you did not say if you needed to face the disks as well. If that is the case, you may want to look at using an internal expanding collet.
Walter