Look for value from on other table to view on the current table base on criteria

Can somebody help me out, i have 3 tables, Transaction Table, source table and Process Table. source table and Process table are related to Transaction table but not to each other. On Souce table i have these fields Transaciton ID, ItemID, Item No, Length, Weight and Size on the Process Table I have Transaction ID, ProcessID, Item No, Paint_Process1, Paint_Process2 and Inspection. On the Process Form in Datasheet view, i have added 3 unbound fields which what i wanted to do is to retrieve and show on that unbound field the value related to "Source Form" for each item no everytime i input them.

Thanks