Hey man,
Sorry because that sounds frustrating. I think that no matter the type you choose, the coach still has a little say, and it may have been influenced by having two other SG's.
What I learned to do to help with that is take people off the lineup for a certain game. When you're setting your tactics, you could remove the other SG's on the roster from that particular game (esp. if they've already got their minutes). Then the coach is less likely to try and sub a different position player in for the SG you're trying to train.
Hope that makes sense,
Sean