if you are going to gang a set of alternators, they will need to be controlled with either one reostat or one regulator, and
you should use fail safe diodes on the output of each alternator, one diode on each output
that way if one alternator shorts out a rectifier bridge you don't have the other two backfeeding and trying to cook the failed unit and in the process flaming the good ones out in the process.
you might want to start with some heavier alternators to start with, maybe that way you only need two?
maybe a pair of 160amp truck alternators? that would get you upwards of 240amps continous,
also you will likely benefit from a large inductor or as they are called in welders
"reactor" this will stabilize the arc and take some of the surge off the alternators when you strike an arc.
figure on at least 2hp for every kwatt output, maybe 3 would be better for this application, and make sure to get the rpm up on the alternators so the cooling fans can do an adequate job keeping thing cooled down.
if i were to do this project i would start with two 110-555 prestolite/leece neville alternators, they have a piggy back regulator that is easy to bypass (delco's do not, so don't use them), i would drive them with an 8 groove serp belt and pulleys, and i would turn the alternators upwards of their rated rpm limit of 8krpm so they cool well. i would use two 300amp 200piv diodes (stud mounted on good heat sinks) and field control reostat, a reactor from surplus center (they still have a few left, and i doubt they will be getting more) and drive the alternators with a 20hp engine minimum, using quality drive pulley's and oem truck alternator pulley's. also i would put two (2)amp meters on the alternator outputs to determine for sure they are pulling equal loading or near so.
should make one heck of a welder if done right
bob g