The configuration settings file used to set up BizTalk server to 3e instance communication can be found here by default: <Program Files of BizTalk Server>\Microsoft BizTalk Server\EAI\EAI.Biztalk.Config (i.e. C:\Program Files (x86)\Microsoft BizTalk Server\EAI\EAI.Biztalk.Config).
A particularly useful value is contained in the <Cobra> node. This holds the transaction service URL that BizTalk uses to integrate with the 3e instance. Connection credentials are also stored here.
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<Cobra>
<WebServiceUri location="http://TE_3E_SERVER/TE_3E_INSTANCE/WebUI/TransactionService.asmx" />
<DefaultUser userName="BizTalkUser" password="*encrypted password*" domain="NetworkDomain" />
</Cobra>
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