I would not use 3 8051's with a single crystal as you will end up linking all the oscillator drive outputs and inputs together this will at least upset the capacitance loading on the crystals and I am not shure about the oscillator driver in the 8051. There are two solutions if you want to run all 8051's off the same clock, either use a single TTL output clock module and a reset supervisor chip or use on 8051 as a master with its clock output buffered before driving the other clock inputs and use a port pin (with pull up?)on the master to drive the reset line of the other two note the reset is high active and port pins are high on reset