I know FF sees a triphasic chart to be a third level of higher temperatures beginning at least seven days after the ovulation rise. They also will not call a chart triphasic if you do not have BD marked near your fertile time. Below is a very "obvious" triphasic chart that FF uses as an example.
That's about all I know about how FF detects these charts. I would say if your temps stay the same or higher then todays temp, FF would detect a triphasic chart
I HTH!