validating an attribute using enumeration in .xsd file

You can create your own type, like so:

<xs:simpleType name="rangedInt">
    <xs:restriction base="xs:integer">
        <xs:minInclusive value="1" />
        <xs:maxInclusive value="3" />
    </xs:restriction>
</xs:simpleType>

And then use it like this

<xs:attribute name="code" type="rangedInt" />

See also W3Schools[^]

This will create an integer type that allows the values 1, 2 and 3.
The type of your attribute should then be rangedInt, instead of a regular integer like it is now (probably).