cylinder compression loss

I have a 92 f150 with the 4.9 inline 6 in it it had sat for approx 5 years with
out running due to bad trans so i put lots of new parts on i ( plugs wires cap
batt air and fuel filter rear man on motor and front main on trans and rear
on trans starter iac silinoid and starter ) as well as put a new parts under
neath and once i was all done i started it it was fine everything worked and
drove good besides at about twenty mph and thirty itwould  loss powwer
shutter but once past the speed it was fine and im kinda at a loss here took
it to shop the said i loss all conpression on cylender 3 so my question
before i tear my head off what are all possabilitys