Hello
Follow these steps
1) Double Click on the Field 3 to open the properties
2) In the Data tab -> default Value Click on fx at the end of the field
3) In the formula box Type <concat(field1, " ", field2)> be sure that you map field1 and Field2 to the fields in the datasource.
4)Use insert field or group button to map the fields
5) Select the check box update this value when the result of the formula is calculated.
Click Ok on the window.
Hope this will help you.
Kalyan G Reddy
Infopath MVP 2007 and 2008