What events, conditions and actions I should use for this case?
I found the answer: 

For Event I used:

 1. After updating an existing commerce product

For Conditions I used:

1. Entity has field - Parameter: Entity: [commerce-product], Field: field_stock1

2. Entity has field - Parameter: Entity: [commerce-product], Field: field_stock2

3. Entity has field - Parameter: Entity: [commerce-product], Field: field_stock3

4. Entity has field - Parameter: Entity: [commerce-product], Field: field_totalstock

For Actions I used:

1. 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)

2. 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)

3. Set a data value - Parameter: Data: [commerce-product:field-totalstock], Value: [stock1_plus_stock2_result_plus_stock3]

You can see what I wrote here -> http://ucreateawebsite.com/DRUPAL/dalexgrup/pt