Oliver
The fact that you can ping the loopback on R3 from R1 demonstrates that there is IP connectivity and that correct routing is taking place between these routers. That is good and eliminates the most common cause of problems like this.
Are there any access lists configured on these routers at all? If so please post the access lists?
Do you have R3 configured with no ip unreachable? If so, remove it and see if traceroute now works.
HTH
Rick
HTH
Rick