How many kms your car has done??? Its not normal to have engine open before 200-300k kms, but in some case it is possible. And yes when 2-3 pistons are behaving like this, its better to completely overhaul the engine as you dont open engine everyday and its better to change all piston and its piston rings too (actually because of which cylinder losses the pressure). There is one quality professional workshop in Rashidiya i take my merc, check with them about estimate and time. Mercedes R us: 04 2852800