This another non RC diy project question. I will be running a 3 phase brushless motor with an ESC and servo tester. My goal is to somewhere place an on/off switch, with the pot on the servo tester set to full speed. I just want to be able to go full speed or off, using a manual switch.

Where accessible!!!! of coarse!!!

I got that.lol I meant which wire. Is it safe to just leave the knob cranked up and just put a switch on the power or ground? Or do I need to put the switch in one of the 3 motor wires? Maybe one of the 3 smaller wires from the servo tester? I dont want to fry anything. This is the second time ive posted to this forum and nobody ever really seems to have a straight answer. All I need to do is turn a 3 phase motor on and off via a switch. That doesnt seem like it should be this complicated. How can this many people on an RC forum not know how to do that?

Your question is really vague and this is an RC forum, so it's not that surprising you're not getting much of a response. You also haven't specified how everything is connected together. Any answer would just be guessing.

I'd suggest putting the switch at the power source (you never specified what that was) or just use the switch on the ESC (you never said if it had one or not).

Also, what 3 phase motor are you talking about? Standard brushless DC motors aren't 3 phase AC motors.

I realize that this is an rc forum, I just figured who would know more about brushless motors than the rc community? Anyways, the esc is in the photo and here's the motor specs.
12v Three-phase Inner Rotor Brushless Motor
VoltageC 7.4-16.8V
Maximum current:24A
Maximum power:380W
Maximum speed:52000RPM
KV Value:3100
Winding connection: Y Type 5 Line and around

Power source is a yellow top optima 12v car battery

That is difficult. You can place a switch in the red wire of the servo wire to shut off the receiver/servo's but it will not shut off the electronics within the ESC.

In this case the only solution is a heavy switch/plug in the battery + wire. In the world of airplanes and boats people mount a female T-plug in the case and use a male plug where the 2 poles are soldere together to plug in as a switch.

http://www.maxxprod.com/mpi/mpi-21.html
(Arming switch)

Does the esc have a failsafe? Just put the switch on the signal wire (white) of the esc. The ESC will respond as if having lost signal, so it will likely stop the motor or you can set the failsafe to do this. You will need to test if the ESC will return to providing full speed on a full signal (it might need to return to zero first).

What is the application?

All brushless esc's have to see a .1500 millisesecond PWM signal on startup or it will lock-out the motor.
Run the switch on the battery + , Transformer + or on the yellow wire of a PC power supply. Dial the servo tester to 20 or 30% travel and see if it calibrates correctly. Mark that spot and return to it every time you startup the system.

Is there some reason you won't run a Tamiya silver can brushed motor with a toggle switch?

I would first like to thank everybody who took the time to try n answer an outsiders question. I just think I need a different servo tester. This one seems like more of a permanent governer. It has some sort of protect feature and if I dont follow the starup procedure, it will not run the motor. the procedure is as follows

Initial start up or after motor disconnect- Adjust pot to max and then connect power wait for the beeps and then immediately adjust the pot to zero.
All following startups- Adjust pot to zero and then connect power wait for the beeps and adjust speed as needed.

That sounds like the ESC causing the behaviour. Usually we don't want our models taking off at full speed as soon as they are turn on, so the ESC will wait for a zero/neutral signal before allowing anything else.

As others mentioned it would help to know what you are doing, why it needs to be a high power brushless motor and why it needs to be be go from off to on at full power.