It seems to me you totally isolated the problem from a possible “radio issue”. According to you, if there is a problem, it’s not due to the radio or TRE. (unless 3 TRX are faulty..? not probable)
All other BTS elements than the TRE are common, so the problem should also happened on BCCH.
So (in my opinion) it leaves three possibilities..
1. RSL issue (SUM card, Abis nibbles allocation)
Are those 3 TRE located on same RSL than the BCCH TRE ? Can find a problem on the signalling link on the Abis ? Can the dynamic Abis allocation provok some disturbance ?
2. TCU issue (BSC)
Are those 3 TRE located on same TCU than the BCCH TRE ? If it’s a different TCU, maybe there’s something going on there.
3. N7 signalling (MSC)
I’m not very familiar with this part .. i leave it to you 🙂
Thanks for sharing this problem here and its solution (when you’ll find it !), sorry i can’t help much.
let us know how it goes,