Thirty-five years ago I installed a programmable timer in place of one switch of a pair of 3-way switches that controlled a single outside fixture. An always hot was available in this box. (Probably the instructions said the timer must be installed in the box with the always hot.)
I think it may be that the box with an always hot will also have a neutral, but there will certainly be a neutral if the line feed enters at that box. If the line feed is entering at the box of the controlled light fixture it may be that a neutral will not always be available.
The instructions were to use a supplied jumper across a particular two of the three contacts of the other switch. And the instructions claimed that despite being jumped in this way the second switch would function as a 3-way switch.
Cannot the new smart switches be used this way? In any pair of 3-way switches one box will have an always hot so put the smart switch there and leave the old switch in place and use a jumper if necessary. New smart switches may not require a jumper like my old programmable switch did.
When I did this 35 years ago I was perfectionist and doubted that the puny jumper (looked like #18 stranded wire) would be the best practice. I realized that the jumper turned the 3-way switch into a 1-pole switch so I replaced the 3-way with a 1-pole switch. (I put two wires under a single screw of the 1-pole switch.) The circuit worked perfectly as if there were two 3-way switches.
This timer (which also was a two position dimmer) was only rated for incandescent lights, and when I switched to LEDs I removed it and put the original 3-way mechanical switches back in. I actually reinstalled 50-year-old mechanical switches in my house!