<p:inputText name="field2" id="field2" type="text"
required="#{searchBean.SearchCriteria.field1==null and  searchBean.searchCriteria.field3==null}" value="#{searchBean.searchCriteria.field2}"></p:inputText>

<p:inputText name="field3" id="field3" type="text"
required="#{searchBean.SearchCriteria.field1==null and  searchBean.searchCriteria.field3==null}" value="#{searchBean.searchCriteria.field3}"></p:inputText>

    <p:commandButton id="search_button" ajax="false"  title="#process="@form" action{#searchBean.search()}" />