Well, I managed to do this, but it's a bit tricky 🙂
%your_search%
| eventstats first(A) as firstA
| streamstats count as cnt
| foreach A, B, C
[eval sub_<<FIELD>>=if(cnt=1, firstA-<<FIELD>>, null)]
| addcoltotals labelfield="cnt" label="newrow"
| foreach A, B, C
[eval <<FIELD>>=if(cnt="newrow", sub_<<FIELD>>, <<FIELD>>)]
| fields - cnt, firstA, sub*
If your initial search is a quite light, you can use append command. The cons are that you have to duplicate your initial search in append's subsearch:
%your_search%
| append
[%your_search%
| eventstats first(a) as firstA
| head 1
| foreach A, B, C
[eval <<FIELD>>=firstA-<<FIELD>>]
| fields - firstA]
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