Hi,
I have a 3d Timeline event which has different instruments in different audio tracks.
I´d like to some of them to keep the 3D panning depending on the position of the listener, but another instruments, i´d like to play them as a regular stereo, not dependant at all of the position of the listener or the emitter.
Thank you 
Just remove the spatializer on the master track and put one on the tracks (or a return bus track), where you want it to be applied.
By the way, it’s an interesting setup!
Just as Alcibiade says, the 3D panning is applied by the spatializer effect. If you remove the spatializer effect from the master track (which is a submix of all the tracks in the event), and add spatializer effects only to the track(s) that you want to be spatialized, you’ll get the effect you want.
Of course, every effect in the event increases the amount of resources it consumes, so if there’s multiple tracks you want to 3D pan, you could save a small amount of resources by routing all those tracks into the same audio track and adding a spatializer effect to only that track.
You may already know this, but I mention it because it’s a common point of confusion: The menu item you use to create an event just determines the event’s initial content, and doesn’t otherwise affect the behavior of the event. FMOD considers an event to be 3D if it contains at least one spatializer, scatterer instrument, or built-in parameter, and 2D if it contains none of those things, regardless of whether the event was created using the “New 2D Timeline” or the “New 3D Timeline” command.
Similarly, the only difference between the events created by the “New 3D Action” and “New 3D Timeline” menu items is whether the new event starts with an action sheet or a timeline sheet. You can freely add or delete sheets from events once they’re created, so an event that starts with an action sheet could end up with (for example) a timeline sheet, multiple parameter sheets, and no action sheets, and would behave no differently to an event that had had started with a timeline sheet and then had multiple parameter sheets added to it.