f = ['Mike	-3,434.09	testing	testing	123', 'Mike	-3,434.09	testing	256', 'No	operation	here']
lines = []
for line in f:
	split = line.split('\t')
	if len(split) > 3:
		tmp = split[:2]                      # Slice the first two items
		tmp.append("_tab_".join(split[2:]))  # Append the rest joined with _tab_
		lines.append("\t".join(tmp))                # Use the updated line
	else:
		lines.append(line)                          # Else, put the line as is
 
for line in lines:
	print(line)

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