define a string in spring context

I have three (A,B,C) spring context.xml, A is for the basic configuration, B and C import the A.

In a bean on A I have:

<bean class="com.example.Ex">
    <property name="aString" value="${myString}" />
</bean>

now I want to define the property myString on B and C context, is possible to do it without create and loads two different properties file?