EOS 3 in newer camera (newer technology) have newer processing chip (for mettering and autofocus, more sensitive cross focus point(s) with slower lenses - 5,6), newer shutter mechanism, it is said it has one of if not the best mettering among EOS cameras, smaller and lighter than EOS 1, I have one and I am happy with it.
On the other hand, EOS 1 is workhorse, it is supposed to be more weather (rain) ressistant than EOS 3, and it suppose to be better built. I didn't used it, but I have photojournalist friends who used it, and were happy with it.
EOS 3 has more bells and whistles, like eye controlled focus (linked to each of 45 focus points ot to groupped focus points), 45 AF points (of course you can reduce that to 11 or to only 1, or to groups, or...

), up to 8 point spot mettering, spot metering connected to one of 11 eye controlled focus points, etc...
Best is: download and read manual for both cameras and have in mind that 1 is heavier workhorse, probablly better built and weather resistance, and 3 is smaller, lighter and more modern.
And, there is more than 10 years difference between 1 and 3, that is if you go for EOS 1 you will get more than 10 years older camera for same money. That don't have to be bad, but it probably worked more, older and more used mechanic in it (more films through it), more lenses on and off camera (lens mount wearing)...
People I know who have EOS 1 wouldn't change it for EOS 3, and I wouldn't change my EOS 3 for EOS 1.
Good luck