Gather the Necessary Tools and Parts

• Spark plug socket (size: 14mm thin wall) and extension
• Torque wrench
• Ratchet wrench
• 5mm Allen key or socket
• Replacement spark plugs (ensure they are compatible with the 2015 BMW 3 Series)
• Dielectric grease
• Antiseize compound (optional)
• Protective gloves and eyewear

Prepare the Vehicle

• Ensure the engine is completely cool before starting the process to avoid burns or damage.
• Open the hood and disconnect the negative battery terminal to prevent any electrical shorts. Use a wrench to loosen the clamp and set the cable aside.
• Remove any engine cover if applicable. This might involve unclipping plastic covers or unscrewing fastening bolts.

Access the Spark Plugs

• Remove the ignition coils to access the spark plugs. For this BMW model, each cylinder has an individual coil pack on top of the engine.
• Using a 5mm Allen key or socket, loosen and remove the bolts securing the ignition coils.
• Carefully disconnect the electrical connection from each ignition coil by pressing the release clip and pulling the connector away from the coil.
• Once disconnected, gently pull up on the ignition coil to remove it, exposing the spark plug underneath.

Remove the Old Spark Plugs

• Attach the spark plug socket to a ratchet wrench with an extension, and carefully lower it over the exposed spark plug.
• Loosen the spark plug by turning the wrench counterclockwise. Be gentle to avoid damaging the threads in the cylinder head.
• Once loose, carefully lift the spark plug out of the engine with the socket.

Install the New Spark Plugs

• Before installing the new spark plugs, check their gap. This should already be set to factory specifications, but it's wise to verify.
• Apply a small amount of antiseize compound to the threads of the new spark plug to prevent it from seizing in the cylinder head in the future (this step is optional as some manufacturers advise against it).
• Place the new spark plug into the spark plug socket and carefully lower it into the spark plug well.
• Hand-tighten the spark plug initially to prevent cross-threading.
• Use a torque wrench to tighten the spark plug to the manufacturer’s specified torque setting. For the 2015 BMW 3 Series, this is typically around 23 Nm (17 lb-ft).

Reinstall the Ignition Coils

• Apply a small amount of dielectric grease inside the ignition coil boot to ensure good electrical conductivity.
• Carefully position the ignition coil back over the new spark plug.
• Reconnect the electrical connector to the ignition coil until you hear a click.
• Secure the ignition coil in place by tightening the bolts with the 5mm Allen key or socket.

Finalize the Process

• Repeat the above steps for each spark plug and ignition coil.
• Once all spark plugs are replaced and the ignition coils are securely installed, reattach any engine covers that were removed.
• Reconnect the negative battery terminal, ensuring the clamp is tight.
• Start the engine to verify that everything is running smoothly. Listen for any unusual noises or misfires.

By following these steps, you should be able to successfully change the spark plugs on your 2015 BMW 3 Series. Always refer to the vehicle's manual for specific instructions related to your vehicle's configuration.