Here's another solution using SEARCH that doesn't make your mind go into a double-negative vortex when doing multiple IF statements. It uses the fourth argument for SEARCH, which returns a specific value if it doesn't find the string, rather than an error:
Hi Guys, I'm a bit late to the party but I had a similar problem and found a solution using =SWITCH(TRUE() and the SEARCH() funcion as @stkbailey showed that works very well. For more information on the SWITCH(TRUE(),***) function, check out the posts on PowerPivotPro about it The Diabolical Genius of “SWITCH TRUE”.
Field = SWITCH(
SEARCH("A", [column], 1, 0) > 0), "A",
SEARCH("B", [column], 1, 0) > 0), "B",
SEARCH("C", [column], 1, 0) > 0) && NOT(ISBLANK([column])), "C", -- Example to show how to add multiple conditions
I will use this pattern many times in the future so Thanks stkbailey for your answer and thanks Rob Collie for your blog.