Reading and responding to data flags

Helllo! I've been pondering how to go about this for a while with no luck. I have a 550x4 table of data. Column 3 is just the recordered temperature. Column 4 is the data flags. There are three different flags accounted for here. There are formated in way such as having no flags reads ",," and for instance only having a flag of the second type is ",D," and a flag of all 3 types is "A,G,F". Im trying to read through all of column 4 for flags and if the second or third flag reads (the first flags are fine) D, G or F (examples ",G,F" or "A,G,D") then the data is not good and so the corresponding row in column 3 should be changed to 'NA'.

I'm not exactly sure of a good way to go about this so anything will be helpful, thank you in advance!