sentences:  The same three companies that built the TXE1 developed TXE3, namely STC, AEI and AT&E, and it was intended to be the standard BPO system for large exchanges.The prototype exchange was built and tested in the Circuit Laboratory at Armour House.The trial period was for 200 subs, 100 were for senior engineers within Telecoms HQ the remaining 100 subs were transferred from Monarch exchange, in the City of London, on a temporary basis (via c/o switches in case something went wrong).