3 hours should still be pretty hot with that high of finished temp, as long as it's insulated fairly well. I've had them resting for over 4 hours with larger packers and was too hot to handle, while with individual flats they will cool a bit faster due to not having as much mass to hold the heat.
You could stick a probe into it when you get ready to process it just to be sure of internal temps...if above 135*,should be no issues at all regarding food safety, as 135* is the minimum recommended holding temp, per 2009 food code. Even if it falls below 135*, it's a matter of the time it was in the danger-zone before serving that really counts, and it probably won't be more than an hour or so, worst-case.
Let us know how it comes out. I'd be interested if you were able to slice or had to pull instead...200* is a pull-able temp for most anything I've smoked, providing it was a good pulling candidate (with a reasonable amount of inter-muscular fat and connective tissue). BTW, I have pulled brisket flat more than once already...pretty cool stuff...very long strands of meat, almost like spaghetti, only a larger diameter and full-length strands when hand-pulled vs shredded with tools.
Here's a mix of pulled point and pulled flat...the flat is longer strands while the point is more of a chunk form...all hand-pulled here:
Again, let us know how you made out...post some pics if you can, too!!!
Eric