I need 3 flashes for shooting in the field. Must be able to kill the sun.
And.....go!
Canon/Nikon (can't remember which you shoot)
If you want auto flash functionality:
Go for one of their top range and two midrange flashes for support. That should give you a master and two slave flashes (though nikon also have their build in master flash on camera as well to consider). If you are feeling richer you can go for all three being top range flashes
If you only want/need manual power control you can consider a wide range of 3rd party alternatives and I'll let others recomend various models of flash.
Also remember you'll have to have some way to control all the flashes together. For canon (the system I know) there is a commander module, but sadly all the canon remote flash control is done through infra red - which seriously limits positioning of the flash units.
Personally I would say go for a set of pocket wizards of the M2 variety as that gives you a faster sync speed and auto flash shooting abilities with either nikon or canon flashes.
The older original model pocket wizards however will be perfectly fine for regular manual power output shooting.
In addition on the budget line you can consider radio poppers or cactus triggers as alternatives to the pocket wizards.
I recommend the radio setups because it really frees how and where you position the flashheads, rather than the infra red or flash signals options which will limit you more so. In addition wired setups are possible, but that can be a messy thing, especially if you are not working in a studio (where you can at least run tape over the cords to keep them from being tripped over).