Microcontroller 16F877A control 6 SCR bridge rectifier

Current I want to use microcontroller to control the 6 SCR. but the source voltage is in 3 phase. Can someone teach me how to let the three phase source go into the microcontroller?

u can use zero cross detector, look datasheet AVR 182

You have to use zero crossing signals from all phases.
for this purpose you have to drop the phase voltage by transformer upto 12 to 15 volts AC then further drop this by resistor network then some digital circuit to make trigger pulse up to micro-controller voltage level when AC voltage crossing the zero line these three pulses are fed to some micro-controller inputs.
and 6 outputs are produced very sharp pulses say 10 to 50 u sec pulses.
one complete cycle take 20 Milli sec. to complete for every SCR you have 10 Milli sec.
to control the power. if you produce trigger pulse after say 5 Milli sec delay counting that phase zero crossing then you are allowing 50% power to the load.if you delay more it will reduce more power.for delay purpose micro-controller timer is used.
this delay is calculated by Analog input of micro-controller.
by variable you control 0 to 10 volts and these voltage are fed to micro-controller input by reducing 50%.
Hope it will help.

This is how I drive a 6 thyristor bridge using PIC microcontrollers. The AC line is fed into the PIC's through a chain of resistors, 6 meg in total.
this gives a nice square wave on the input to the PIC. The PIC's then sort out the firing of the thyristors depending upon the phase rotation, and the input from the feedback circuit.

Regards
Bob...