<MyType>
    <tag1>Something</tag1>
    <tag2>Tag 1</tag2>
    <tag2>Tag 2</tag2>
    <tag3>Another something</tag3>
</MyTpe>

<xsd:complexType name="MyType">
    <xsd:all>
        <xsd:element name="tag1" />
        <xsd:element name="tag3" />
    </xsd:all>
    <xsd:choice maxOccurs="unbounded">
        <xsd:element maxOccurs="unbounded" name="tag2"  />
    </xsd:choice>
</xsd:complexType>