Make resin copies of the parts, use those on the models, and put the plastic bits back in the box with the original. I have a hard time justifying buying three Sealabs just because I need parts for three models...but that's just me.
If you look hard enough you find that a lot of kit leftovers are used on other models from the same movie / tv show.
I've only had 1 model where almost the whole single kit is used, it was the AMT Silhouette which if you rearange the parts becomes a police car used on the old Thunderbirds tv show. It's also the only studio scale minature I've built that requires only 1 kit
I tend to build the kit if possible without the part(s) I used.
The Monogram B-29 is such a kit
If the kit cost a ton of money I cast the parts needed and then sell the kit after returning the original. Think Bandai Hummel or Otaka D51-101 which are over $100 each.
If the kit is used in a lot of models the parts are cast and the originals are placed in a "parts libary" to remake molds as needed. Think Airfix Saturn 5 or the 8rads.
I keep everything as stock for future kit bashing needs. I also make resin copies and I treat their molds like gold so they last long. Even while building my SD, when I make copies of a part I need multiples of, I will at the same time make molds of other interesting parts from the same kit that I can see using on future projects. My basement looks like a mini ILM model shop and studio.