sensors confused

[hahaho]

I want to use the sensor of the three.
these sensors want to fixed number[A,B,C], (no value[x,y,z])

for example)

sensorA(fixed number) = (10,10,10) <-x,y,z (receive(change) value)
sensorB(fixed number) = (20,20,20) <-x,y,z (receive(change) value)
sensorC(fixed number) = (30,30,30) <-x,y,z (receive(change) value)


so, i am used this code.

vrpn = viz.add('vrpn7.dle')
sensorA = vrpn.addTracker('PPT0@166.104.29.228',0)
sensorB = vrpn.addTracker('PPT0@166.104.29.228',1)
sensorC = vrpn.addTracker('PPT0@166.104.29.228',2)

def update(): 
      position1 = sensorA.getPosition()
      position2 = sensorB.getPosition()
      position3 = sensorC.getPosition()
     
      a =  position1[0]
      a1 = position1[1]
      a2 = position1[2]
     
      b = position2[0]
      b1 = position2[1]
      b2 = position2[2]  

      c = position3[0] 
      c1 = position3[1]
      c2 = position3[2]
      
      A = [a,0,a2]
      B = [b,0,b2]
      C = [c,0,c2]
      
      # vector
      v1 = vizmat.VectorToPoint(A,B)
      v2 = vizmat.VectorToPoint(A,C)
      v = vizmat.AngleBetweenVector(v1,v2)
      Forearm.setEuler([0,0,-v])
      UpperArm.setEuler([0, -30, -80]) 

      print v
 
vizact.ontimer(0,update)

but , the sensor's number is frequently change.

for example)

sensorB = (10,10,10)
sensorA = (20,20,20)
sensorC = (30,30,30)
.
.
.
sensorC = (10,10,10)
sensorA = (20,20,20)
sensorB = (30,30,30)
.
.

How do i receive the fixed sensor's number?