This is something we have had in particular on table this week within the design team. Let’s walk it thru, there is a small story for this:
The housing design of the phone is super tight, and it even started to look like we would be able to fit only a single hybrid tray that would share the 2nd nano-SIM with the microSD. This without increasing the device size.
We pushed further and after several iterations we’re able to find a physically smaller (and slimmer) battery on the same capacity, i.e with a higher density. This free’d some space (and yes, we’re talking about parts of a millimeter in case of thickness) so we could revise the design and get more space for the 3-slot SIM tray (which is naturally bigger than a 2-slot tray). However, this does not mean the design would be able to fit separated trays or locations for dualSIM and microSD, that won’t be feasible. And not even speaking of a 3.5mm audio jack here 
So, thanks to slightly slimmer battery which enabled to revise the design, a 3-slot tray can fit to the housing. Yay!
Thus, there will be one dual-sided tray having two nano-SIM slots and one microSDXC slot. I.e. in total 3 slots within one tray.