I've searched an searched, doing some math.. but it seems a bit complicated here.
Does anyone have a clue to create a trimetric camera?

Don't get confused with Isometric or demetric because isometric has 3 axes and demetric 2 axes that have the same angle to eachother. In Trimetric view, 3 axes have different angles from eachother.

My question is,
How do i create a trimetric camera in unity?

You could attach a script to your camera in which you create a projection matrix so that it gives a trimetric projection, and assign it in place of the camera's projection matrix (already calculated from either it's a perspective or orthographic projection and with the relevant properties).

Uhm, won't an orthographic camera already be trimetric just by itself?

To get an isometric or dimetric projections off an orthographic camera might actually be harder than a trimetric one, since you don't have to match angles...

I'd just set my camera as orthographic, pitch it down some 60 degress, and set it to about 60 degrees off center, so it looks at stuff from an angle...

Now, if you want to have full control of each angle on your projection, the matrix is the way to go.