I like all three of these machines. Any changes I'd make would be niggles, and mostly a matter of scale. I'd be happy to win any of them, and would probably do some minor mixing and matching with what I already have.
For all of them, I'd upgrade the SSD to 500GB just because I can.
I'd pull the power-scarfing R9 390 out of the gaming PC, put in my GTX970, and probably add another drive for a RAID1 pair. I'm not anti-AMD, but another 200W+ under load is a horrific waste. I really hope AMD can get back in the low-power competition; the HD4670, HD5670, HD6670, and HD7750 were all amazing cards for how little power they used.
The prosumer machine would get a RAID1 pair of drives added.
The LAN box is actually a tough call. I could just add one more drive to it also, but I'd be sorely tempted to put a H170 mITX board and Skylake i3 in it instead, if only because those are available to me. I would not need to though; it's an excellent machine as it is.