How to have a static ISO and shutter speed, but automatic aperature. in video mode.

What camera are you using ?
I just tested on my 7D I can switch it to TV set shutter and ISO.. And It will change aperture by itself in video mode.

But You would not what to do it like that. Because When it changes aperture it does it in steps.

SO mine went from 2.8 to 5.6 and not a smooth way. that gradually darkens the image. More like bright to dark like flipping a light switch. That's what the video looks like.

you get better results by having iso on auto. you would then get slow gradual changes tothe scene. but if its too dark you get noise with higher iso.

Or a manual lens with declicked aperture ring .