3 year old always sounds congested, mouth breaths and snores at night what could this be?

There are various: possible reasons for this: enlarged adenoids and/or tonsils, nasal congestion from allergies or an upper respiratory infection. Have your child's pediatrician take a look and they may also refer you to an ear,nose & throat specialist.

Snoring: he may have big tonsils and adenoids see an Ear Nose And Throat specialist surgically removing the adenoids and tonsils will help your child