Field Calculator - Python - using 3 fields for parameters get syntax error

I'm trying to create a model to do field calculations using 3 fields for parameters. I get the attached syntax error when I do. I'm successful in using 2 fields as parameters but when I add the 3rd, I get the error. Is there a limit or am I doing something else wrong?

Other suggestions for doing field calculations which could be easily modified by another.

Attached is the field calculator w/ code and error.

Any help would be very much appreciated. Thank you!

Attached Thumbnails