"'angweDDaq QongtaH wej 'avwI'pu'."
Translation:The three guards were sleeping in the museum.
The three guards were sleeping in the museum suggests that you are giving a narrative that concerns three guards, and it is somehow relevant that they were sleeping in the museum.
Suppose, however, that there were six guards, three in the museum and three in the garden (Du'Hom), and you want to specify that it was the three guards in the museum who were sleeping, as opposed to the others.
In such a situation, I think we would say, in English, The three guards in the museum were sleeping. In such a case, the in the museum is an adjectival phrase modifying guards, as opposed to an adverbal phrase modifying were sleeping.
As @jdmcowan points out, adverbs go at the front of the sentence in Klingon. To my mind, this means that the sentence angweDDaq QongtaH wej 'avwI'pu' really corresponds to "[The] three guards were sleeping in the museum".
If you wanted to say The three guards [who were] in the museum were sleeping, I think you'd have to say in Klingon something like QongtaH angweDDaq chaHbogh wej 'avwI'pu''e' or, perhaps more less literally, QongtaH wej angweD 'avwI'pu' (The three museum guards were sleeping.)
It's OK. In English you can move "in the museum" to a number of different locations within the sentence. There are subtle differences in the implications of different positions, and sometimes Klingon can mirror those subtleties, but this isn't one of those times. The locative can ONLY go at the start of the phrase in Klingon, so all of the English placements are represented by this one Klingon phrase (or by longer more complex explanations to clarify those subtle distinctions). I've added in that variation now.
I'd disagree -- I'd consider "the three guards in the museum" short for "the three guards who are/were in the museum", where "in the museum" qualifies and identifies "the three guards", rather than saying where they were sleeping.
I'd rather translate "The three guards [who were] in the museum were sleeping" as something like QongtaH 'angweDDaq tu'lu'bogh wej 'angwI'pu''e'.
(or perhaps chaH(taH)bogh, as Michael.Lubetsky suggests; I'm not sure whether that works.)