You seem to be saying that you are working 12 hours and only being paid for 8, yet you also state that you are given 3 and 4 hour lunches and lunch breaks are not paid lunches. Your shift can be split if that is what is taking place and here's some step 4 grievance settlements that describe how this should work.
M-00224 Step 4
January 27, 1982, H8N-1N-C-23559
1) When a part-time flexible employee is notified
prior to clocking out that he should return within
two (2) hours, this will be considered as a split
shift and no new guarantee applies.
2) When a part-time flexible employee, prior to
clocking out, is told to return after two (2) hours,
that employee must be given another minimum
guarantee of two (2) hours work or pay.
3) All part-time flexible employees who complete
their assignment, clock out and leave the
premises regardless of intervals between shifts,
are guaranteed four (4) hours of work or pay if
called back to work. This guarantee is
applicable to any size office. See also M-00982,
M-00246, M-00576, M-01405
Look these up at:
http://nalc.org/depart/cau/pdf/mrs/mrs0104.pdf