Can I default on my 2nd and 3rd mortgages if my 1st mortgage is more than my house is worth?

Technically the 2nd or 3rd can foreclose, but in reality they won't because as you say, the 1st is more than the home value. Also, if the 2nd were to forelose, it would have to payoff the first, since the 1st would still remain. So in reality you can stop paying your 2nd and nothing will happen. However the lien will remain against the property, and interest will continue to accrue on the balance. It is also possible the 2nd lender will just charge it off, but again the lien remains.