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 :)