YES they can mix DOT 3 and DOT 5 without any problems. if you mix the 2 type of fluids it will not be much better than the poorest quality fluid.
there was a post on this subject a while ago that had the federal law stating they had to be mixable.
when i change over to DOT 5 i just remove as much DOT 3 out of the master with a suction device. then i pour in the DOT 5 and bleed until i see the DOT 5 at the wheel cylinders. after i do a 4 wheels i bleed out about 1/2 of a master cylinder full at each wheel and call it a day. i usually use 1 to 1 1/2 quarts of fluid to do this.
one thing to remember is DOT 5 retains air like DOT retains water. i like to place the DOT 5 in the hot sun for a hour with the cap off to help solve this problem.