Unless you have an interrogator (like a TCAS for the office), the old modes will require several receivers to do multilateration. It's complicated by the fact that Mode-A and Mode-C don't identify as such. They are just bits, so you have to use a bit of logic to put them in the right table.
If you had 4 receivers, 4 antennas, you could probably get a 3D display.
It would be cheaper to equip your fleet with ADS-B and then you get all sorts of good data, like GPS position, altitude, velocity over ground, etc.