I think, those three phoneticly similar words have been replaced in many regions with the "urban" form, "Dere".
So, "There they are!" would actually be; "Dere dey are!"
They're, is slightly different, having the form "Dare".
Other words that have succumb to the "D" treatment are;
That, becomes "Dat"
This, becomes "Diss" (also used as "Diss", when referring to a lack of respect).
Them, becomes "Dem" (as in "dem bones, dem bones).
They, becomes "Dey".
Though, becomes "Doh".
These, becomes "Dese".
Notice how the accompaniment of "t" and "h", seems to have been almost universally replaced, with simply "D".
When these combinations are put together, particularly with a Scouse accent, they can becomes quite amusing;
"Dey don't talk like we do, do dey doh?"