Some transition words are specific to a particular part of a paper, but most are not. If the third paragraph is at the end of a paper, you can use:
If the third paragraph is in the middle of the paper, you can use:
A third point is...
If the third paragraph is in the middle and is where you are presenting a contrasting idea or refuting objections, you can use:
On the other hand
While some people believe...the truth is...
In general, the most important way to choose a sentence starter for any paragraph is to look at how the information in that paragraph relates to the information in the paragraph before. Is it contrasting? Adding? Extending? Concluding? Use a transition which shows the relationship.