Copy on write is an interesting idea used on server systems. Does it make any sense on a smartphone? The data pages that are actually modified need to be copied. This approach, called copy on write, improves performance by reducing copying.

According to me it is unnecessary as every process has its own data. (not sure) A third process working with that data is really not possible.

Please tell me if it's possible, if possible then why? Also an example where this will be helpful would help.
Thank you.
Yes, copy-on-write makes sense on smartphones, too.  Smartphones are just computers, packaged to be small.  Smartphones run conventional operating systems, with multiple processes.  For instance, each app typically runs in its own process.  And yes, these processes do use shared memory, shared libraries, fork(), etc., so copy-on-write is useful (and is supported on typical smartphone operating systems).