I pronounce
through as "throo", and
slough as "slow", with the "ow" spoken to rhyme with plough (US: plow). I pronounce
though as "thoe" to rhyme with toe and dough. For me, the three sounds are quite distinct, but I don't think there is a consistent rule about
-ough that makes them so. I just had to memorise them arbitrarily as part of my ESL training, where we were aiming for "educated standard Australian" pronunciation.
I wanted to sound like Cate Blanchett...
As for "the Beast":
Edit: It occurred to me later that
slough can
also be pronounced "sluff", as in a snake sloughing off its skin. English is so weird.