Thanks to everyone for the feedback. Looks like I'll be lacing that Promax / NuVinci build with the 2x pattern. Just confused on why my spokes ended up being too long (3-4mm) on that first electric wheel build that I posted a photo of. I used an online spoke calculator and made measurements of the hub using a pair of digital calipers. However, this was my first time doing a 2x (using Musson's method) wheel and it was very difficult to measure though because of how large the flanges are. I'll double check all my measurements and the lacing pattern.
It sounds like there's no real scientific way to check whether a 3x will work on any particular hub/rim combination. Just have to use my best judgement based on the flange diameter and ERD of the wheel.