Greg,
I've always used 3rd gear when doing a burnout with my Jericos ( both Duster and AMX) I've never had 3rd gear break and I do everything that everyone tells you not to do. First, I make sure they have enough water in the waterbox- not enough and you could glaze your clutch. I dump the clutch at 7000rpm to make sure that I don't bog the motor in the waterbox and then bring it up to 8000rpm while spinning ( i like to hear the motor beg for mercy) I smoke the heck out of the tires ( way too long and burnout) and then let go of the linelock and as I'm driving out of the box, I keep the motor at 8000-8500 until the motor starts to pull down as the track grips ( you don't want to be up against your rev limiter while doing a burnout as a harmonic vibration can setup and kill your valve springs). This is pure abuse on my part- I'm sure you don't have to rev it as high or spin the tires as long.
I check my valve spring pressure and set lash before each race ( not each run) My clutch is set a little tighter than most keep theirs, so I don't have trouble with a glazed disc but I do eat a ring gear every 60 or so runs in the Dana. I would not be afraid of hurting 3rd gear if your clutch is set on the loose side. My tighter clutch setup is much more likely to hurt 3rd, but I never have.