[jrob]

I have seen some definitive guidelines on several matters, number of blocks,etc.
In a 15x15 how many 3 letter words are too many?
And on that, for many years while driving when i look to my GPS I hit the NAV button. But that's just my car. Do you think I can get away with NAV as a 3 letter word?

[worldofcrosswords]

I don't know the answer to the question about 3 letter words.

I checked the database (http://www.cruciverb.com/data.php), and NAV occurs many times. Mostly, it has been clued as an abbreviation for "Naval". There are a couple of more recent entries that clue it in the sense that you suggest. That sense hasn't appeared in the NY Times so far.

Marya

[XMAAS]

There's no hard and fast rule about 3-letter words. However, in general, the fewer the better. Unless the puzzle is otherwise exceptional, 20+ is pushing it.

As for NAV: go ahead and clue it how you wish, since the word is OK anyway. The editor can always change your definition.

-MAS