exact one word translation for all the three is 'MY'
mera is used when the gender of the object in ur possesion is masculine and singular
ex.
Mera ghar
(my home),here ghar is m
mera friend.
Now it should be clear that i m talking abt my friend who is boy/m.
Meri- it is used when object has femenine gender and it may be singular/plural
ex.
Meri kitaab.
My book.
Kitaab is f in hindi.
Meri friend.
My frnd(who is female)
same rule applies when object is plural.
Mere-
1. it is used when we talk abt masculine plural object
mere ghar.
My houses.
2. Whenever object is masculine singular and there is preposition (we should rather say post position, because they are placed after word in hindi) in sentence, mera changes to mere
ex.
Mere ghar mein.
in my house.
Mere ghar se.
From my house.
Mere ghar ko
to my house
etc.
3. Addressing someone(m), exclaimation
mere dost!
My friend!