legal depth is 1.6mm
they seem to be ok, but picture is shit tbh!
first, your gay for buying 2nd hand tyres... you will never know if they are already damaged untill its too late, and they go bang bangwhen your on the motorway! good eh.
just because there is no visual sign of damage, this doesnt mean the structure of the tyre hasnt ruptured in anyway! which can be done from curbing, pot holes or incorrect fitment.
the top one, i would hazard 4mm... put a 5er on it

the other 3, 2, no more than 3mm.
give it 1000 miles and you will need new ones, depending how you drive.
alot of town driving, and driving hard, give it 700.