Sum only numeric values in range (and disregard cells with inactive formulas)

I'm looking for a formula that will sum the numeric values of three cells (K16, K17, K18). The tricky part is that each of those three cells is using a formula to populate its number. Simply adding them up or using the sum function doesn't work if any of those three cells are not calculated. I want to get the total sum (in cell K20) using whichever cells are populated with a number (even if it's only using 1 or two cells). K20 should also be blank or "N/A" by default (and not 0). I've uploaded a sample workbook. Thank you for the help!

Thanks for your response, CARACALLA - it's certainly better now. However, I'm finding that the total store store isn't calculated when Healthy Aisles and/or Checkout points are missing. Any thoughts on how to account for that? (This is basically a store audit form and I want to make sure I can still get the score even if someone forget to fill out a section.)