Question

1. discuss: to talk about something with someone and tell each other "your" ideas and opinions

2. discuss: to talk about something with someone and tell each other "his" ideas and opinions

3. discuss: to talk about something with someone and tell each other "their" ideas and opinions
--------------------------------------------
The definition of 'discuss' is in #1. What about the others? Can't we use 'his' or 'their' instead of 'your'?

Answer 1

"their" is inappropriate. "each (other)" is singular, "their" is plural. Either "your" or "his" is okay. "your" is the same, singular and plural, "his" is singular.