Double Clicking Cell To Select Choice Data For Another Cell's Formula

Hey Guys, So I'm new to this forum and I hope my process for inquiry is accepted, please let me know if I make any mistakes.

So, I have a formula in B9 that is dependant on the choice between A3 and B3. I don't want to have to type in the information these cells hold into the forumula for B9 every time, I need to be able to double click one cell, and have that signify what the formula is in B9. Keep in mind, the information in Cells A3 and B3 is text, but should signify two crucial differences in Formula for B9.

Thanks in advance!!