Lyman alpha question

I was wondering if anyone could clarify my following question about
filter band redshift calculations for me. From other sources I am
informed that redshift interpretations from simultaneous optical
observations in different filter bands can be made by working out which
filters the source is visible and which it isnt and then calculating a
redshift approximately from that data. So for example...
If filters U,B and V are used to observe a light source known
seperately to be at z=3 am I correct in assuming that due to lyman
alpha cutoff no detection could be made in the U band filter as the
lyman break prevents this ie..121nm*3(+1)=484nm. So anything blueward
of 484 nm isnt visible.? Or in other words am I correct in thinking
that if one were viewing in U, B and V filters one would get no
detection in U (area curve 300-400nm), a possible detection in B ( area
curve is 380-490nm) and a definite detection in V band (area curve
500-590)?
thanks
Sean