You say your machine is 120v and 5kw output. Neither of these figures are likely to be more than approximations so you will need to alter things about somewhat.
Taking your figures you need heaters that will load 5kW at 120v and that will be your full load.
As soon as the wind drops the load will be too high and you will stall the prop.
In low winds you must have no load so it will start up. When you reach a useful wind speed ( perhaps 10mph ) you need a load that will match the machine output at that speed. As the wind picks up you need to keep increasing load so the prop doesn't run away and you do this up to the maximum power.
If you are clever with electronics you have many choices but if you are not, the way you are most likely to be able to manage is the way that someone has pointed you to in the link to Hugh Piggot's site at Scoraigwind.
This adds the load in discrete steps and has some dead band so that the loads don't keep cycling. It is still likely to beyond the capability of relays long term so you need to think about electronic relays ( triac based).
The final step requires that you don't exceed the heater voltage so if your 120v is nominal you may have to use higher voltage heaters.
The more steps the better you match the load but 3 steps should give you a working system.
You should keep the load balanced round the 3 phases, particularly at the higher wind end.
It is simpler to rectify to dc but you need mosfet or igbt pwm controllers.
In theory you could use a 3 phase controlled rectifier ( diode /scr) but you will find no commercial firing circuits for variable frequency.
The same is true of ac phase control using triacs.
Hope that gives you an idea of what is needed, how you do it depends on your skill.
Flux
