“Trans” is not a label for a third gender that one might have instead of and to the exclusion of man or woman, “trans” is (like, and in opposition to, “cis”) relationship between gender identity and “sex”—actually, ascribed gender—assigned at birth; it can coexists with any of man, woman, nonbinary, or other (or, in the agender case, no) gender identity.

Though your not knowing what “trans” means very clearly explains your not understanding the need for “cis”.