Multitrack - Recording from internal computer stereo input AND 2 mics (Using ext audio interface)

Hello all,

I've looked around a lot for answers to a problem I'm having but to no avail so am posting here.

I have 2 mics connected to my computer via USB through an external audio interface. I have no trouble in audition setting up/recording on seperate tracks for these. However, I also want to add a third track that is recording internal computer sounds (I.e Spotify). By now you can probably guess where I'm going with my project. In any case Audition will only allow me to set one audio hardware default output (That being either the stereo output for the computer sound, or the output for the audio interface capturing the mic sounds).

It seems I can only record one or the other, I've played around a bit and unable to resolve. Does anyone have a quick explanation for how to properly set this up? Happy to explain further if I've butchered this hah!

Any help is massively appeciated!