Hi @Opelayo80,
The advice we can give you on the Community is going to be relatively limited. I'm not clear from the post if you're still abroad, but you'd be best to report this to the local police service, let them know the IMEI of the phone if you have a record of this.
If you get in touch with customer services, they can assist you in blocking the SIM and phone. Blocking an IMEI won't necessarily apply to networks outside the UK unfortunately, but it's best to do this regardless.
If you have the phone insured with a 3rd party, we can assist with a document called a proof of usage. If you didn't originally purchase the phone with Three, you may be asked for a proof of purchase from when you originally bought the phone in order for us to give you a proof of usage.
I hope this helps,
Jonathan
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