The 2.5A recommended for the 3B/3B+ models includes 1.2A for the USB ports, which leaves 1.3A for the Pi3 itself.
So 1.3A should be enough if the PSU has good voltage regulation, and that assumes no accessories. Anything on the USB ports, or any added HATS or other accessories need to be included in the power budget. As stated above, 1.3A is worst case, fully loaded, so you might be able to get away with less if the system is not going to be pushed hard (and if you're not maxing it, you might consider a more power frugal model).
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?