Let's presume I have a link in my page 1. After clicking on it, I want to navigate to page 2, where I have an iFrame and load some content (page3) I specify in page 1. Any kind of advice is greatly appreciated.
Your question is incredibly vague.
If you wish to make a link go to the next page it's as simple as using the href="" property like so:
<a href="page2.html" >Link Title</a>
Then is you have the iframe in page 2 already it'll load, just make sure your root path to page2.html is correct in the href section.
For more conclusive or indepth answers then rewrite your question with code examples of your attempts and clearer description of your goal. What you want to achieve can easily be found with a quick google
Assuming you have a page 1 with different links and all of them take you to page 2, and the only thing you want to change when clicking the different links is the content of the iFrame, you could use POST or GET (depending of what information you are passing, take a look at this.
Page 1:
<a href="page2.php?link=1">page2, content one</a>
<a href="page2.php?link=2">page2, content two</a>
And then on page 2:
<?php
if ($_GET['link'] == 1) {
$iFrameURL = "url1.html";
} else {
$iFrameURL = "url2.html";
}
?>
<html>
...
<iframe src="<?php echo $iFrameURL ?>"></iframe>