A belief is an internal feeling that something is true, even though that belief may be unproven or irrational. eg I believe that walking under a ladder brings bad luck, or I believe that there is life after death.
A value is a measure of the worth or importance a person attaches to something; our values are often reflected in the way we live our lives. eg I value freedom of speech, or I value my family.
An attitude is the way a person expresses or applies their beliefs and values, and is expressed through words and behaviour. eg I get really upset when I hear about cruelty to children and animals, or I hate school.