How do i record splits if im using my phone to record my 3ds.

well, you don't need splits to speedrun. so you can just run a timer on your phone for reference, or on your pc, or use a stopwatch.

You don’t. Not unless you manage to capture both your splits on a computer and the 3DS in one video on your phone (which is unlikely).

Just film the game, splits are pointless for submissions.