japanese police and their law in general is not only extraordinarily harsh (hence the low crime rates) but extremely expensive for anyone considered a suspect.
its a lot like legal extortion, you pay for everything, even access to your lawyer, phone calls to/from family etc, but no matter how much you pay it wont effect the outcome of your case. even your own lawyer will be in on it, cant pay extra? too bad you get a real estate lawyer instead of a criminal defender, and the whole time your own lawyer will be trying to make you confess anyway.
its often better for innocent people to claim guilt than fight a charge.
if anything by denying involvement the third guys family will end up financially broken and he will get the longest sentence.