An "exact" transformation of your code as written might look like this:
<<set $var to either("a", "b", "c")>>\
<<if $var is "a">>\
<<if $var is "b">>\
<<if $var is "c">>\
However, I'd recommend some modifications. First, for something like this, I'd recommend you use numbers rather than letters. It doesn't really matter, but it seems like an odd choice, since numbers are generally easier to track and require less typing.
Further, your code as written does not require the $var at all if you use the <<switch>> macro instead of the <<if>>:
In the above code, we use the <<nobr>> tag to suppress line breaks. We then use the <<switch>> macro to evaluate the random() function, which we've set to return a random integer between 0 and 2. If the number comes back 0, it trips the <<default>> clause (sort of like an else) and that <iframe> is shown. If it comes back 1, that <iframe> is shown, etc.
Some other notes:
You could also probably assign the iframes classes in your css to handle the styling so you don't need to rewrite all the style rules every time. It's also possible to use a bit of jQuery to randomize just the src attribute of your iframes so you don't need to rewrite it at all, but I'm not sure it'd provide any benefit outside of that, and isn't really necessary.
All things being equal, I'd use the <<switch>> if you can, as saving on the variable and using the more succinct syntax is probably worth it.
Moved <<default>> to the end. Sorry about that.