not "feng san bi ying", but "feng san bi du". if the computer has three on the line like " *** " and if you do not place one at either ends of it, next step one more will be place at the line and it becomes " **** ". You definitely lose.
"suan san" means like the following:
**
*
*
If you place one more at the corner, it becomes
***
*
*
No one can stop you gaining the 50 silver la.