Without any idea what the intended use for the feeder is, it's
impossible to
give advise, however if its a clean straight shot, three 3\0 conductors will
fit. Just make sure you don't try pulling it with the nylon strand

The intended use is to power a workshop with a 4 post lift, air
compressor, welding equipment, and a wood shop (mostly 220v). It is a
one person shop generally, so it would be unusual to have more than one
load at a time, though my so has welded while I sanded or lifted a car,
and with the present hopelessly inadequate feed it browned out the
entire shop.
I had every intention of using the nylon to pull the wire, glad you
mention that, what should be used? I have pulled cars with that stuff,
I thought it was bullet proof.