A Hallow is usually regarded as another name for a saint. Halloween is another name for All Hallows Eve, the next day being All Saints Day. As to regards the 'Deathly' bit...guess we'll have to read the book!
[ive just finished reading the book today so...]
as to the Hallows, they're 3 objects combined with Dark Magic to allow power to the owners
so: 3 objects, 3 locations, 3 functions
"deathly hallows" is refered to as this, simply because death comes to those who possess it