#define HAVE_STRUCT_TIMESPEC

 #include<bits/stdc++.h>

 using namespace std;

 vector<char>ans;

 char a[][];

 int main(){

     ios::sync_with_stdio(false);

     cin.tie(NULL);

     cout.tie(NULL);

     for(int i='';i<='';++i)

         ans.emplace_back(i);

     for(int i='A';i<='Z';++i)

         ans.emplace_back(i);

     for(int i='a';i<='z';++i)

         ans.emplace_back(i);

     int t;

     cin>>t;

     while(t--){

         int r,c,k;

         cin>>r>>c>>k;

         int cnt=;

         for(int i=;i<=r;++i)

             for(int j=;j<=c;++j){

                 cin>>a[i][j];

                 if(a[i][j]=='R')

                     ++cnt;

             }

         int x=cnt/k,y=cnt%k;

         int pt=;

         int tx=x;

         if(y)

             ++tx,--y;

         for(int i=;i<=r;++i){

             if(i&){

                 for(int j=;j<=c;++j){

                     if(a[i][j]=='R')

                         --tx,--cnt;

                     a[i][j]=ans[pt];

                     if(!cnt)

                         continue;

                     if(!tx){

                         ++pt;

                         if(y)

                             tx=x+,--y;

                         else

                             tx=x;

                     }

                 }

             }

             else{

                 for(int j=c;j>=;--j){

                     if(a[i][j]=='R')

                         --tx,--cnt;

                     a[i][j]=ans[pt];

                     if(!cnt)

                         continue;

                     if(!tx){

                         ++pt;

                         if(y)

                             tx=x+,--y;

                         else

                             tx=x;

                     }

                 }

             }

         }

         for(int i=;i<=r;++i){

             for(int j=;j<=c;++j)

                 cout<<a[i][j];

             cout<<"\n";

         }

     }

     return ;

 }