Only calculate average when 3 people have given vote

bot.go

 
 			}
 		}
-		if strings.Index(line, AVERAGE_MARK) != -1 {
+		if strings.Index(line, AVERAGE_MARK) != -1 && count > 2 {
 			expression := fmt.Sprintf("'''{{#expr:(%s)/%d round 2}}'''", strings.Join(scores, "+"), count)
 			newLine := "| " + AVERAGE_MARK + " " + expression
 			changedLines = append(changedLines, newLine)