I have submitted a paper to a conference, but some one in lab help me after the paper has been submitted. Can I add them as a third author if the paper got approved to be appear in the conference ?
I think that in almost all cases this would be fine. But you should contact the conference (or program) committee chair immediately to get confirmation. Better to do it early than to be disappointed later.
This would especially be possible if the conference gives you a chance to revise the paper after review and comments by reviewers. Many do this.
It really depends on the conference and their advertised submission rules in the Call For Papers. Some top conferences have strictly stated rules and it is impossible to change the authors after the paper submission. They state this clearly and in the submission online system editing the authors fields are disabled after the deadline. However, some conferences allow adding authors after submission. I think you can figure our this from the submission system. If it allows you to change the authors after submission, and there are no written rule about this in the Call For Papers, then you can change. Otherwise, you may try to contact the program chairs. But if it is a top conference, and they have announced their rules in the submission instruction with the Call For Papers, chances that you can add an author are very low.