-Our is a possessive pronoun, it refers to a group of people where the
speaker is included, in relation with an object in property:
"Our car(WE BOUGHT THAT CAR)"
-Ours is a possessive adjective, the meaning is the same than the
previous but the type of word and position in the sentence is different.
"THAT BIKE IS OURS"
- Us is the Direct Object form of the personal pronoun `We`. It is used
when a group of people where the speaker is included receives the verb
"The teacher warned us for not doing the homework""