group       Values
  0         aa_bb_25_0_1
  0         273
  0         60
  1         aa_bb_7_0_2
  1         335
  2         aa_bb_65_1_1
  2         456
  3         aa_bb_98_0_1

group  Values        Modified.values
      0  aa_bb_25_0_1  aa_bb_25_0_1
      0  273           aa_bb_25_0_1
      0  60            aa_bb_25_0_1
      1  aa_bb_7_0_2   aa_bb_7_0_2
      1  335           aa_bb_7_0_2
      2  aa_bb_65_1_1  aa_bb_65_1_1
      2  456           aa_bb_65_1_1
      3  aa_bb_98_0_1  aa_bb_98_0_1

awk -F"t" -v OFS="t" '$3!="" {three=$2} $0!="" {$3=three; print $0}' test.tx