=AVERAGE('panel-mv3'!BP2+'panel-mv3'!BP104+'panel-mv3'!BP206 ... +'panel-mv3'!BP8876)
=AVERAGE('panel-mv3'!BP3+'panel-mv3'!BP105+'panel-mv3'!BP207 ... +'panel-mv3'!BP8877)
=AVERAGE('panel-mv3'!BP4+'panel-mv3'!BP106+'panel-mv3'!BP208 ... +'panel-mv3'!BP8878)
...
=AVERAGE('panel-mv3'!BP103+'panel-mv3'!BP205+'panel-mv3'!BP307 ... +'panel-mv3'!BP8977)

for ($i = 1; $i < 103; $i++) { // 102 dates
  echo "=AVERAGE(";
  for ($j = 1; $j < 101; $j++) { // 100 companies
    echo "'panel-mv3'!BP" . ($i + $j) . "+";
    echo "'panel-mv3'!BP" . ($i + $j) + ($j * 102) . "+";
    echo "'panel-mv3'!BP" . ($i + 1) + (($i * 2) * 102) . ")";
  }
}

<?php

for ($i = 2; $i < 104; $i++) { // 102 dates
  echo "=AVERAGE(";
  for ($j = 0; $j <= 99; $j++) { // 100 companies
    echo "'panel-mv3'!BP" . ($i + $j*102);
    if($j!=99) echo "+";
  }
  echo ")n";
}