Rather late reply, so I don't know if this is still of use to yourself (although hopefully it'll help someone else if not)!
I've got an NB3 and use it for exactly what you suggest, SHO imaging with an OSC camera, combined with an l-extreme for Ha / Oiii. The best I can work out the band passes for both Oiii and Sii on the NB3 are rather wide, which doesn't appear to be widely advertised, I found the following chart whilst looking into the possibility myself:
This also shows quite nicely why the NB3 requires stacking with a second filter to remove the UV/IR wavelengths, I've been using an l-pro for this purpose because it has higher transmission at Oiii/Sii than my cheap UV/IR filter but I imagine something like an Astronomik L-3 would be a better choice.
Unfortunately due to the wider band pass of the NB3 compared to the l-extreme I find myself not using the Oiii from the NB3 when processing images, as it generally just reduces the SNR compared to the 7nm Oiii from the l-extreme alone. I imagine though if you were using the NB2 and NB3 together as "intended" by IDAS this wouldn't be an issue. For this reason I've been tempted to switch to a dedicated 6-7nm Sii filter instead, although I did notice recently that since I bought my NB3 Askar have released a 6nm Oiii/Sii dual band filter too.
https://www.firstlightoptics.com/narrowband/askar-colourmagic-2-6nm-oiiisii-duo-band-d2-filter.html
If you're interested in some examples I've got a few uploaded on my Astrobin account:
NGC 1499 / California Nebula (Starless SHO)
NGC 7635 / Bubble Nebula (SHO)
Melotte 15 / Heart of the Heart Nebula (SHO)