Another option is to try to find an old and super cheap laptop and use that instead. Maybe find one with a broken screen or busted keyboard or something that you won't care about.
This is essentially what my MBP with butterfly keyboard has been relegated to in life. The keyboard is unusable, it's too expensive to repair, but it's not a shitty enough computer to just throw away, and there's practically no trade-in value either. So, there it sits in a closet, on a top shelf, just acting as a remote device doing random things as I assign it tasks.
However, cheap is not how I would have described it ;-)
If your laptop's USB ports will provide enough amps, that could work.
Doesn't the Pi 3 expect up to 2.5 amps? I imagine some of that is for powering USB ports, so you could probably get away with 1.5 amps if you're not using them, but even that's a lot for laptop USB. I'd be surprised if any offered more than 1 amp.
Laptops basically have a UPS built-in.