Typically, you don't. You have your X3 winched onto a flatbed, cringing the entire time. I can pretty much guarantee you that whomever is providing BMW Roadside Assistance in your area knows nothing about the procedure for forcing your X3 into Neutral.
Got power? Engine cranks but doesn't start? I haven't tried it because I've never been in a condition where my engine won't start, but this is the alleged procedure:
1. Depress brake pedal
2. Press start stop button...engine won't start but will crank
3. Press shifter release button on side of shifter and hold it
4. Move shifter forward one step-hold it there for 2 secs....exactly
5. Release side switch and repeat. N should be displayed in instrument panel...trans unlocks
Got no power, engine won't crank? Now it becomes a bit of a problem.
Got a wheel chock, a jack, a jack pad, and a jack stand? Got tools? If not, you don't have a way to safely shift into neutral.