We started with 2 cats (one has since passed away) and both took the harness pretty easily. First thing to remember is that the harness (get an "H" type) is foreign to them so put it on and then ignore them for a day or so. Then take them out, attach the line and sit down and wait for them to walk around. Don't force them to walk. Just wait, and when they do, then follow and keep the line slack.
Now, if you have 3 cats that can't get along, then I would start looking for new families for them as being in a smaller area and moving around is upsetting enough without adding in not getting along.
Barb