Can I Hook Up RX-460 to Regular PCI Slot via PCI-E Riser I have motherboard with 3 Regular PCI slots beside 3 PCI-Express slots. (https://www.asus.com/Motherboards/M4A88TDV_EVOUSB3/specifications/)
Can I Hook Up 3 RX-460 to Regular 3 PCI Slot via PCI-E Riser USB?  Will it be stable?
No.  PCI and PCI-e are two entirely different things.  PCI is a more traditional parallel data channel, while PCI-e is a serial data channel (that can have multiple parallel "lanes").  

You cannot plug PCI-e devices into PCI sockets.

If you're desperate, you can try the mechanism specified here, no guarantees though :)

https://www.youtube.com/watch?v=5vDHACwXSxE