Python coding in Field Calculator. Nested If statement

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I'm trying to use field calculator to assign values to a new field I created. I wan't to assign the value from the first field unless that field is empty, if it is empty use field 2, if that is empty use field 3. Here is what I have thus far. I feel it is way off but not sure where to go from here, any help is appreciated.

New_Size: My new field

wmDeviceEq: First field to check for new field name

wmDevice_4: Second ""

wmDevice_5: Third ""





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