Am i entitled to have share

You stated that the first wife and third wife are legally wedded. How is it possible?  However, all the children of your father have equal rights over the property.

Yes both are wedded accordingly,

My question is even though wedded the third wife legally, When the first wife is alive ? they say the marriage is void or null,

I such case how will I get my share if my mothers marriage is null or void.

Now the first wife and her kids are telling me that my mothers marriage is void so I wont get any share

The other wives kids are just scaring u. Don't worry. Courts have stated in many judgements that even when the marriage is void, the kids can demand equal share as a coparcener. Go n get ur equal n full share as u hv full right to do so

Which religion i.e., Hindu / Muslim / Christian etc. your father professed ?

Except Muslim subsequent marriage during life time of wife (FIRST) is void and children born out of such arrangement are illegitimate, even than they have a share in the property owned by their father.

Seek professional guidance of a local prudent lawyer for better appreciation of facts and necessary proceeding.

You are concerned with only one issue:  Your right on the ancestral property as the son of the third wife.  Be assured you are having equal rights with others.

All the legitimate or illegitimate son's and daughter's of your father are entitled to get eaual share in ancestral property of your father.

How is that the property is ancestral? Is your father alive?