First of all, this has little to do with MVC3.
Depends on many things, especially the authentication. You
can[
^] consume SOAP web services from ajax. You could pass credentials too (ntlm/kerberos should be transparent), but that raises several security issues, since you should never store plain text (or even digested) credentials on client side.
I suggest you move this functionality on server side and present this to the client under the cloak of the regular normal services provided by your server application. This is the only reliable approach if clients are spread over the internet, and there is no guarantee that the client will be able to consume the third party service in a reliable manner.