That's... not true. Trying to be #3 doesn't mean you say "OK, we won't get any customers from the first two, so all we have left are that 15%" it just means you don't think you can take enough of their customers to overtake them.
And to people saying it's bad for company moral... it really isn't. Very few companies can be #1 or #2 in their industry, and for everyone to think otherwise is more often than not just delusional.