I'm looking to rectify 240 VAC into DC with 100 amp surge capacity.
The largest I found is 50 amp [...]
Do I need to come up with some way to put three in parallel? If I do
this do I need to fuse each rectifier?

If a 100 amp unit is out there, my life would be simpler.

I've always heard that diodes don't parallel very well, I think
because they won't always trigger at the exact same time, so whichever
triggers first will take the brunt of the current. Even if a fuse
blows, the remaining diodes are now taking more than what they are
rated for. And as the old saw goes "the diode will blow to protect
the fuse."

I'd be tempted to take a page from the guys throwing together bridges
for getting DC out of AC-only welders, and use 4 individual stud
diodes...? Apparently if you get two pairs, one pair with the stud on
the anode, and another pair with the stud on the cathode, it's easier
to assemble. Then at least in theory, every component would be
capable of sustaining the full current it is exposed to.

Maybe you would you be able to find a suitable bridge as a welder
replacement part? Though finding one rated for 240V RMS (350V or so
peak) might then be an issue...

Just my $0.02,
--Glenn Lyford