On page 117 of Shadowrun Third Edition, it states that tracer rounds can be mixed in with other ammunition types as a special exception. On page 280, it says they make it "easier to home in on the target". However, at no point in the book does it say what they actually do mechanically. Is there a book where this is detailed?
The previous page (SR3 p. 279) tells you where to look for all the specific bullet rules - Ammunition in the Combat Section (SR3 p. 116).
Tracers are loaded every third round, and reduce TNs by 1 per three rounds fired at any range beyond Short (for everybody except smartgun users, anyway). However, the tracer rounds don't count for the Power increase you usually get when firing bursts; a three-round burst from a 5M weapon would be 7S instead of 8S.
They don't explicitly detail what to do when you fire a multiple other than 3, but I advise not worrying too much about it. Tracers are most effective in FA weapons, but those are generally the purview of sammies, who are going to have a Smartlink anyway.