Very easy,
First decide which bit of your 8051 port will drive which led segment.
and make a chart on paper.
Then fill in the chart to show which segments to turn on for each
digit pattern.
Use the pattern data to program a lookup table for each digit (0-9)
if you require a blank (all segments off) program one of the unused
locations (A-F) for this.
To display the digits access the digit pattern in the table and write
it to a ram location,(MOVC A,@A , MOV A@R0) Write all 3 digits
to 3 ram locations.
Then move pattern data in ramlocation 1 to the port ,turn on the digit-1
driver for a period of time maybe 100ms , turn it off load next pattern
turn on digit-2 driver again 100 ms, then digit-3 the same and loop
back to digit-1.
That is the general idea , I hope this is clear enough.
Bruce