This has been linked here before, but...http://www.mototuneusa.com/break_in_secrets.htm
I found this lnk ages ago when surfing. I followed this method when I broke in my new engine. Because my new motor A15, is bigger than the old a14, (129cc's) I jetted it really rich.
I towed the car out to the countryside, and let rip.
Once it was warm, I pushed my brand new engine, up to 105mph. (On the clock) Being towed home, I phoned the tow car driver to check my speedo, and it turned out it was reading slow by 15%. So I had pushed my BRAND NEW engine to 120mph @7PSI.
No problems. That was holding back a bit too!
It sounds like your engine builder should know his stuff. Did he bore some extra clearance into the bore? My machinist gave me an extra three thou, to allow the piston to grow a bit more from boosted tempratures.
The suggestion of a gas anyliser is a good one. I recently borrowed from a mate a broadband air/fuel ratio meter. I put a boss on the tail pipe to fit a five pin bosch O2 sensor. You plug it in, and tape it up the boot and in the window. Then the remote plugs into the cigarette lighter socket. It's got a digital display which tells the a/f ratio.
It told me I was running rich as. I used it to diagnose why it goes flat at 6000.( Leaning out, not a spark problem)
If you can get a hold of one of these, you'd find it most helpful.

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It doesnt have to show marks to say lean out. Detonation is violent.
Think about cylinder compression pressures and add detonation. Thats a lot of hard banging going on.
Now think about all the things that can contribute to causeing detonation.

thats what i mean,i know detonation is violent, so if it was detonation ud think there would be damage on the top where the detonation occurs not on the lands, and i would think it would occur in more places other than this one small spot plus, thought it would do damage to the rings. this is the only damaged piston and the rings on all pistons werent touched, im no expert but i think detonation would occur on all the pistons not just one.... ohh wlel at the end of the day doesnt matter the engine is blown next time i wont be doing the same again. it will be tuned by a professional with proper equipment.

The rings arent fixed in place, they can move alittle.The rings are pushed against the lands during piston movement. You give something a good rattle and the weakest part will eventually fail.

I just had a thought,
You're running sidedrafts right, could there have been a carburation problem with the venturi that feeds number three cylinder?

what sort of pistons are they???

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broken ones!!! lol.