Best: Record *in the cameras*, then bring all of those files into a video editor later. No OBS at all, and no capture card. Might need to tweak the time-alignment slightly and periodically, because the clocks are not synchronized, but every edit is an opportunity to do that without anyone noticing.
Or perhaps the editor can time-align everything for you with very small percentage of speed tweaks, so you don't have to worry about it after that. Mine does:
Shotcut is a free, open source, cross-platform video editor for Windows, Mac and Linux
shotcut.org
Okay: Set up each one as its own source in OBS, then use the Source Record plugin to put a filter on each one. That filter then records that point in the processing chain, to its own file, all simultaneously...provided that your storage device(s) can keep up that you send them to. (recording 3 videos simultaneously is 3x the datarate as recording just 1)
Download Plugin for OBS Studio to add a filter that allows you to record a source. Add "Source Record" filter to your source. Installation Copy and merge the folders that are in the download zip to the OBS folder for example: C:\Program...
obsproject.com
