1) Film frame rates: First, create an NLE timeline set to the output resolution you would like, eg: 30fps 1080p. ie, a single reference point. Then simply drag all media onto the timeline just created - one video track for each of the three camera ‘angles’. That takes care of everything - all will be previewed and used according to that single timeline reference point. No need to covert frame rates etc etc.
One caveat ‘may’ be in relation to different video codecs & some NLEs handle different sources better than others. But really, these days most NLEs should be able to cope. You may also use the NLE’s internal transcoding features to change everything to one format & for speedy processing purposes if you wish, Adobe Media Encoder for example, but is usually time consuming and disk space intensive because of the larger, edit friendly codecs like Pro Res or DnxHR. I’d be inclined to avoid that for the moment & the NLE should be able to handle native .h264 codecs in the timeline.
Frame-rate and sample-rate: Anything you want to set for the common timeline really, but I’d probably avoid drop frame for simplicity (is only US broadcast-centric really). 25fps PAL, 24fps film & 30fps are all common. Smaller frame rates also mean smaller final renders. Also depends on delivery destination. If for personal /web use etc, use whatever is conventional in your home country, perhaps 30fps /48kHz should do it.
In terms of final render, you can always change your mind and ‘deliver’ for any number of formats, screen sizes and frame rates. That is a function of the NLE output render options, not the input formats.
2) Video sync: You will need to sync all three video clips in the NLE timeline, depending on if you have same or different video start points for each of the three cameras. You can also use timecode or time to sync; NLEs have various automated features to take care of that (more or less). Another option is to just eyeball the wild video audio waveforms and sync them up (I assume the cameras all recorded sound of some sort). Useful to have a slate tone or handclap at the beginning of each video to make this simple. If not, there will likely be a common ‘strike’ or note somewhere early where you can also do this.
3. Premier: You mention using this NLE, so – go read the manual and/or study some of the video tutes about editing camera angles (multi clips). Also the sync tools as per above. Personally, I would suggest checking out (free) DaVinci Resolve, for everything here.
4. Cubase:. Create a project in the same frame rate as one of the clips and perhaps use the ‘wild sound’ for visual reference purposes. Set the sample rate the same as the NLE project, say 48k. Mix and export the finished audio mix. Frame rate is irrelevant here for audio export, just the sample rate. You may not even need to bother about loading a video clip. When mixed, export, load & sync the mixed audio to the reference clip(s) in the timeline. Eyeball should be fine, as per already sync’d clips above in (2).
Hope that helps. Is not nearly as complicated as you suggest. A single NLE timeline destination format is the key.