Yes, it will be doable. You might have to mix and match using both the dock and system (depending on the exact dock series), but it looks like if you use ports on the system you can technically get this setup as long as the switchable graphics are enabled (and it should be).
Here is also a link discussing this. I got the info above from internal sources, but you also might see this on the sales/support site.
Thanks Brad. Looks like I will need the newer port replicator to get DisplayPort 1.2 support. I've ordered and will report back when I receive it.
I promised an update, so here it is:
I picked up the "Dell E-Port Plus Advanced Port Replicator with USB 3.0" from Amazon, which has DisplayPort 1.2 support (If you already have a port replicator, check that the USB ports on the back of it have a blue tab in them. Blue USB ports mean that you have the port replicator with USB 3.0 support which is the one necessary for DisplayPort 1.2 MST support).
Video port 1 on the replicator is plugged into one of the U2715Hs (which has MST - labeled as DisplayPort 1.2 in the monitor's settings - enabled) and then another one of the U2715Hs is connected to that monitor via daisy chain.
Video port 2 on the replicator is plugged into the 3rd U2715H.
Things seem to work for the most part. All three monitors are running at 2560x1440.
The only problem I am having at the moment is that the monitor with MST enabled will occasionally appear to lose signal temporarily (screen goes blank) and then it will come back. At this point a fairly minor annoyance but I haven't spent a lot of time looking into it. Could be a Windows 10 issue, but I don't know yet.
I take some of that back. Both of the monitors in the daisy-chain configuration flicker (lose signal) frequently after a few hours of use. Either this configuration isn't supported, or there is a bug somewhere.
Let me see what I can find out on why the MST is flickering, but in the meantime can you try running the 3 displays without the daisy chain? 2 of the monitors from the dock (1 from each display port on the dock) and then one plugged into the Precision itself (HDMI/Mini DP)? I am pretty sure that will work for 3 displays, but just wanted to verify this with you.
I am now running that configuration and it appears to be working. It is less than ideal, so I would rather not have to run it this way long term. FWIW, I'm running Windows 10, the latest version of the NVIDIA drivers, and all dell updates applied.
FWIW, I have read anecdotal accounts that suggest changing the power plan to "High Performance" can resolve this issue, but in my testing that appears to have no affect.