You had a good go, but grammar always makes things complicated right?
Well, milli is a better word for between, and it takes the genitive, so Hel and the deep sea have to both be in genitive.
Hel - Heljar
Djúpr - Djúps
Haf - Hafs
and Eða is actually "or"
Ok is "and".
So here it is:
"Milli heljar ok djúps hafs" - "Between Hel and the deep sea"
"Þrjú skaut til vinds" - "Three sheets (ropes) to the wind"
Skaut also refers to the rope meant by "three sheets" in this case.
I personally frown upon Elder Fuþark used with Old Norse... Younger Fuþark is what goes with Old Norse... but it's still popular to use Elder Fuþark instead.
Well in Elder Fuþark, you would write them as:
:milli:heljar:auk:djups:hafs:
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:þrju:skaut:til:winds:
But if you can persuade them to use the correct fuþark. The Younger! It would be:
:mili:hiliar:auk:tiubs:hafs:
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:þriu:skaut:til:uints: