One piece race suit for everything (under a full wetsuit obviously for the swim). Thrown on some arm warmers for the bike and take them off for the run. That is all I would use.
Of course, I ran a 15K this am and it was 48 degrees and raining (shorts and shirt only and I was fine). I hate being hot on the run, but I like to be a little warmer on the bike.
That MAY be enough. Three other things that I would do that take less than a minute to do would be to pull on some knee warmers, gloves and a helmet liner( thin toque).