Personally, I have no issue with the way transgenders present themselves in society. It actually shows a lot of courage to stand up for who you feel you really are and what you believe in. They are still human beings and aren't any more a threat to others as male or females are.
People are free to be who they are. They should be free in choosing their sex or gender. No one is able to control their likes or wants. They should be who they want to be. If they want to be a gay, then they can. If they want to be a lesbian, then they can. No one can stop your likes or wants.
Have we attained perfect equality between men and woman already? We're a long ways away from getting two genders to treat each other well. Adding a third gender is like adding a third party candidate to a two-party presidential race--the third party candidate usually gets decimated. Labeling people as a third gender will make it easier for society to discriminate against them, far more so than the opportunities to bring about equality. This is why most intersex societies oppose labeling intersex individuals as third gender. People would use the label more to exclude individuals rather than as a way to be inclusive. If you're a man who like to wear pink, now you're third gender. A woman who gets a job on an oil rig, she's now third gender. It's better to have two genders that are large and allow for a lot of variation than three or more genders that will become increasingly exclusive over time as stricter definitions get applied to who is allowed and who isn't allowed to have the label third gender.
Can we legalize marriage with them? That will be an upcoming question. What is a 3rd sex, I now ask? I am assuming we mean those with no genitiles or male and female genitiles, or perhaps the OP has learned of yet another sex or is creating one in their mind. A 3rd sex could be someone created in a lab I suppose. What are they like is my next question? I'm invisioning someone with male genitiles and femal3 breasts and the ability to give milk to infants but no ability to carry a baby. Another version is one that can carry a baby with male genitles but female reproductive ability through artificial means. Wierd.