During that time, there were 28 different platinum units (including Zenobia). If the chance difference is it being 8 times as likely to find the spotlight unit (though that was noted for black units, and other rarities may be different), and the rest are equally likely options, you have an approximately 2.86% chance to find most platinum units, and an approximately 22.86% chance to find the spotlight unit (assuming a single platinum spotlight, as was the case during that time). That leaves the chance that you don't get her when rolling a platinum at about 77.14%. The odds of not getting her 3 times is about 45.90%, meaning that you would only have had a little over a 50% chance to obtain her in 3 rolls, and as most people can tell you from experience with Farming events, 50% can very easily go either way.