Calculate the sum for 3 CCK fields with Rules in Drupal 7
I found the answer: 

For Event I used:


After updating an existing commerce product


For Conditions I used:


Entity has field


Parameter: Entity: [commerce-product], Field: field_stock1 

Entity has field


Parameter: Entity: [commerce-product], Field: field_stock2 

Entity has field


Parameter: Entity: [commerce-product], Field: field_stock3 

Entity has field


Parameter: Entity: [commerce-product], Field: field_totalstock



For Actions I used:


Calculate a value 


Parameter: Input value 1: [commerce-product:field-stock1], Input value 2: [commerce-product:field-stock2]; Provides variables: Calculation result for Stock1  and Stock2 (stock1_plus_stock2_result)

Calculate a value


Parameter: Input value 1: [stock1-plus-stock2-result], Input value 2: [commerce-product:field-stock3]; Provides variables: Calculation result for stock1-plus-stock2-result and Stock3 (stock1_plus_stock2_result_plus_stock3)

Set a data value


Parameter: Data: [commerce-product:field-totalstock], Value: [stock1_plus_stock2_result_plus_stock3]



You can see what I wrote on http://ucreateawebsite.com/DRUPAL/dalexgrup/pt.