The perfect watch would be one that runs slow so that I think I’m going faster than I am.
I’m pretty sure the 310xt/910xt does everything you mention
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http://www.google.com/products/catalog?q=garmin+forerunner+910&um=1&ie=UTF-8&tbm=shop&cid=4809558180311254610&sa=X&ei=f7S8Ts-9LdG9tgfC6ri5Bg&ved=0CI4BEPMCMAI
https://www.google.com/search?aq=f&gcx=c&sourceid=chrome&ie=UTF-8&q=garmin+forerunner+910#q=garmin+forerunner+310&hl=en&safe=off&prmd=imvns&source=univ&tbm=shop&tbo=u&sa=X&ei=krS8Ts7jC5LNtgfug_jIBg&ved=0CLgBEK0E&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=f8402f01c489f1b3&biw=1680&bih=892
I think a mode on the watch called the “tri” mode
The Garmin Forerunner 310 and 910 do exactly what you are asking. It’s called Auto Multisport on the 310 (and probably will be the same thing on the 910 when it comes out), but you can customize the multisport mode as well, so that you can have it move between swim/bike/run or swim/t1/bike/t2/run or even duathlon or some random adventure sport. On the 310, you can customize your multisport mode, choosing from “Other,” “Bike 1/2/3,” “Run” and putting them in whatever order you need for your event. Buy one, you’ll love it.