If the car is equipped with a wheel speed sensing TPMS system, then fitting the transmitters is of no value. The car will not have the required pick-up antennae and electronics. The vehicle will sense a low tire by measuring a speed difference between the low tire and the other, normally inflated ones. If your X3 currently has rubber tire valve stems, and has a functioning TPMS system you have no use for TPMS transmitters.
For the best (safest) possible situation if you have to drive on a completely flat tire, BMW recommends using "run-flat" wheels (these have an extra "hump" that helps keep the the bead completely seated on the rim under cornering loads). However, several tire manufacturers (but not all) now say that their "third-generation" run-flat tires can be used safely on any rim, as long as they are driven conservatively. I believe Bridgestone and Dunlop are in this camp, while Pirelli, GoodYear and Michelin are noncommittal, and don't say you shouldn’t use plain rims, but they also don't specify that a run-flat rim is absolutely necessary for their tires.
Non run-flat tires can be mounted on RFT rims, it's just a little harder, usually more expensive, and requires a RFT mounting machine.
Since all the BMW wheels have the extra "safety hump", and since it's obviously a theoretically better solution, that's all that BMW recommends.
I have my Winter Dunlop SP Winter Sports ROF mounted on aftermarket wheels (no- rft hump), and if I get a flat will just drive carefully at speeds well below the 50 mph limit as well as corner very carefully. (Fingers crossed)