Taking just a second to think about how a processor samples an external signal,either analogue or digital will show that if you want to sample three inputs simulutaniously then you need some system where the samples can be stored in some form of memory before they can be processed,clearly a processor such as the 8052 can only store one sample to memory at a time so there is no real way that you can take three samples and store them to memory at the same time.What you would need is some dedicated hardware which would store the samples in hardware which could them be read by the processor.