I want to protect 3D charecters which are used as emoji in my chat app, how can I do it: copyright (the Copyright office of USA offer a registration service) or as trademarks?
I am not a lawyer, nor do I have expansive knowledge on the subject, but probably, yes. Both copyright and trademark them (via the USPTO and the Copyright Office.)
Copyright because they are creative works, and trademark because they are specifically your emoji, distinct from others' emoji, and a brand.
A commenter has noted this, which I shall include verbatim:
The USPTO is not essential to the ownership or enforcement of any trademark in the USA -- rights arise from the use in commerce. The Copyright Office does not issue or enforce copyrights, but your copyright may not be enforced in US federal courts prior to filing for copyright registration. 17 USC § 411
Go through this link, it answers this question very well.
Apple has 2d emoticons registered with copyright office, and so should 3d characters be eligible for copyright protection.