Do same trained skills stack?
For example, if I was an Eladrin Wizard, trained in Arcana, would I get a triple bonus from having three trainings in Arcana?
No, you wouldn't. Trained is an ON/OFF condition, you either have it or you don't. Thus, having it turned ON three times is no better than having it ON because of three different features.
On page 31 of the PHB, point 20 says:
Skills: Check off the skills you’re trained in and determine your base skill check bonus for each one. You have a +5 bonus to checks using skills you’re trained in. See Chapter 5.
You check if it is trained. If it is (as I said, ON/OFF), you get a single +5.
No, on the box on your character sheet there is no space to fill it in multiple times. Once you’ve filled it in, you get that +5.