A Triangular Slice of Squared Pi
05AB1E, 19 bytes

Uses CP-1252 encoding.

nžs¦¦¹ôI£íRvyN&gt;£J}R


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Explanation

n=5, y=3 used for example

nžs                  # push pi to n^2 digits
                     # STACK: 3.1415926535897932384626433
   ¦¦                # remove the first 2 chars
                     # STACK: 1415926535897932384626433
     ¹ô              # split into n*n matrix
                     # STACK: ['14159', '26535', '89793', '23846', '26433']
       I£            # keep the first y rows
                     # STACK: ['14159', '26535', '89793']
         íR          # reverse the list of rows and each individual row
                     # STACK: ['39798', '53562', '95141']
           v     }   # for each y,N (row, index) in the list
            yN&gt;£J    # keep the first index+1 digits of the row and join to string
                     # STACK: 353951
                  R  # reverse the string
                     # STACK: 159353
                     # implicit print