Dear Stata users,

I have a data like below, the researchers input variables as alphabet. Now I want to convert those strings into numeric such that "A" as "1", "B" as "2", "C" as "3". It is easy to do when string has only one alphabet, but in cases that strings was concatenated as "A,B,C", how can I address it? Thank you in advance for advice.
Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input str20 x1 str22 x2 str18 x3
"A"     "A"       "C"      
"A,B"   "A,C"     "B,D"    
"B,C"   "C,G"     "A"      
"A,B,C" "C"       "B,C,D,E"
"B"     "B"       "B"      
"C"     "C"       "A"      
"A,B"   "A,C"     "B,D"    
"B"     "E"       "E"      
"A"     "B,C,D,F" "A"      
"A,B"   "B"       "A,B,C,E"
"B"     "A,F,G"   "B,C,E"  
end