A spacetime diagram on rotated graph paper allows ticks to be read off.
Essentially, follow the bouncing light-signals (arranged so that the principle of relativity is satisfied), which bounce off parallel mirror worldlines [not shown].
All light-clock diamonds (which visualize the ticks of a wristwatch) have the same area.
Here is one for the problem you pose.
The light rays broadcast by the initially-forward-traveler
are shown, which can be used to compare what the other observers see with their eyes to compare the remote clock-face with the local clock-face (associated with the Doppler effect).
The most time elapses on the inertial path from separation to reunion.
This diagram is taken from my earlier answer
What if two twins flew off in opposite directions and were reunited in a perfectly symmetric way, would they have aged same?
A more elaborate explanation is at another answer
Question about Special Relativity similar to twin clock experiment
For something with asymmetric motions,
go to https://physics.stackexchange.com/a/508935/148184
from Equivalence of two definitions of proper time in special relativity