a 3 jaw chuck on the lathe for the piston decking is fine...but if you wanted to use the lathe to machine out the bowl/radius (rather than do it freehand) you'd need a 4 jaw chuck...since the bowl isn't centered on the piston.
indicate it round first...then indicate on the face...then recheck round...recheck face...
and machine as necessary.
the piston has a slight taper (.004-.005 off the top of my head)...so you may need to take a thin strip of brass stock and wrap it around the smaller OD
(up near the ring lands).
measure the OD
(to the nearest ten thou) before chucking them up...and double check the OD
after machining to make sure you didn't distort them by over tightening them in the 3/4 jaw...been there/done that.
snug is fine...& wear a football helmet
have you checked the part numbers on the pistons?
if they are Mahle 144200WR0x0 (x being 1,2,3, etc referring to 010/020 etc overbore)...then that is a STANDARD height piston
the Mahle 144201
WR0x0 is a 010 shorter DECKHEIGHT (and the appropriate 010/020/etc overbore)
it's possible that they mixed & matched some pistons...
the only diff is that the "201" part number is 010 shorter from the piston pin hole to the top of the piston...to allow for block machining w/o having to deck the pistons...
but good for you for CHECKING that chit...