Praetorian97,
AFAIK, the only 3-way independently electrically powered trimmers are the Gracey and Giraud trimmers. Both register on the case shoulder. That's a plus in that it gives you more consistent neck length. Both a quick enough to be used every load cycle, so you can safely trim on the long side.
The 3-way cutter is possible minus to the extent your resized cases have necks that have uneven wall thickness or are pulled off-axis by your sizing die expander, as you then get an offset or asymmetric chamfer using these machines (due to the shoulder registration), where a square trim can be offset from the case's longitudinal axis and still look the same. This could lead to uneven bullet pull if you crimp. I've seen no direct evidence of problems from eccentric chamfers using my Giraud, but I don't normally crimp rifle cartridges and I don't use cases with significant wall unevenness or neck runout in precision loads for slow fire, so I don't really get a chance to see if it can affect group size.
Note that in the sizing die, a fired bottleneck case normally contacts the sides of the die before its shoulder reaches the die shoulder. For that reason it first is narrowed and squeezed longer, then the shoulder is formed back to the desired length from the head, flowing the extra brass into the neck. This is what lengthens the neck. So, trimming before resizing will not catch the growth length from the last firing, though extra length accumulated from previous load cycles will come off. But if you want precise control of length, resize first, then trim. Years ago I did some before and after resizing measurements of .308 Win cases from the same lot with the same load history and got a growth spread of about 0.010" coming out of the resizing die. So if you don't wait to trim until after full length resizing, you will have some variation like that. Again, probably of no consequence if you don't crimp, but not a good idea to have if you are trimming your necks to the long side of tolerance.
Finally, on the square trimming, many just use that without chamfering and deburring. The fast cutters don't seem to leave much of a burr. The problem is the sharp inside edge of the case mouth will scrape off bullet coatings and scrapes some copper. Even on some commercially loaded ammo you can identify a small ring of copper scraped up to the case mouth. Bart Bobbit mentioned that can affect bullet symmetry to a degree that affects long range groups. He suggested using an easy-out in a drill to burnish the inside edges of the mouths of freshly trimmed cases, either chamfered (still sharp, if less so) or unchamfered to avoid this. An alternative is to buy and use a Lyman M die to put an small expanded step in the case mouth, same as you would do to load cast bullets. That step allows you to set each bullet into the mouth straight, which causes the seater to seat straight, avoiding runout. This does have a measurable effect on accuracy. Putting the step in also dulls a sharp edge. However, the crimp shoulder of your seating die or a separate crimp die may need to be applied just enough to iron out the step for maximum feed reliability. (In some cases the step barely passes the bullet heel, so you don't see any trace of the step after seating, in which case the crimp ironing isn't needed.)
