Cool, so it does kind of matter where the Red is then :P You can combine some kind of logic. choose(0,1,2) would give you a random selection to give to Red. if Red is 1 then it appears in the middle for example. You can combine that with a function or loop that you run with a counter that repeats 3 times. So a global variable set to choose(1,2,3) with a separate counter from 1 to 3, if counter = gv, then add Red, else pick a random colour from the array.
Maybe look something like :
set gv to choose(0,1,2)
for (loop) 0 to 2,
if loopindex = gv add "Red"
else, run the search through array for a random colour
output would be 3 colours, none the same, with red included at a chosen position (the global variable)