Random & mark

Hello everyone
I want to ask two question about random and mark !
There are three location :
A ( 0 , 0 , 0 )
B ( 100 , 0 , 0 )
C ( 0 , 0 , 100 )
When I start the game , the object will emerge on A or B or C .
What should I do ?

In addition ,
I have to use the following method to set a mark

65207-30.jpg

, so the object must stay in this

What should I do to resolve these problems ?