1) John, Mary, and Sam are best friends. 2) John, Mary, Sam are best friends. In sentence 1, the compound subject used the conjunction "and"(John, Mary, and Sam) and in sentence 2, without conjunction (John, Mary, Sam). Here my doubt is can we use the compound subject without conjunction or it must be necessary for compound subject?
We distinguish two types of coordination: syndentic and asyndetic coordination.
Syndentic coordination is just a normal coordination with two elements being conjoined by one coordinator.
He and his sister are naughty.
That of asyndetic, however, is a rather different in that it takes no coordinator to indicate the coordination.
Quickly, resolutely, he strode into the bank
When three or more items are conjoined, the conventional style is to use one coordinator between the final conjoins.
John, Mary, and Sam are best friends.
Note that I said 'style' not 'rule'. Even if it were possible to use no conjunction here, I would avoid doing so.