Apart of other interface considerations.

a 12 bit AD should be about 20uS for 12MHz device

12*3*20*1000*1000=720000000 microsecons

720 secons per frame 12 frames (2 times each one) 720*12= 8640 seconds 2.4 hours each second of film.....

You need a flash A/D converter. or be VERY patient.

By the other side..
1000*1000*12*3/8=4500000 bytes /frame
You need to traslate at the time of scaning or a 4MB adresses device
If you connect the 8051 througth serial port to PC at 56Kbps takes 8 secons per frame.