Scroll down to the article "Last Chair can be Cool Too!" written by my alter-ego. http://itgyouth.trumpetguild.org/jou...ITGJjr0406.pdf
It explains some of the aspects of playing third, another fun thing is being the bridge between the horns or trombones, so in addition to matching the sound of the principal trumpeter, we get to match the sound of the first horn and first trombone as well. Third players get to play louder than the first, yet remain "hidden." Ninja stuff!
On some orchestral scores, the first and third part are written on one stave and the second on another. At one rehearsal the conductor stopped after a chord and said "we need more third and less second trumpet here." He had us play the chord again, shook his head violently, and checked the score more closely." We need more second and less third trumpet here!"
Have fun and kick butt!