Blame Nader if you will, but Nader didn't blow 3 debates, for Gore. Gore didn't challenge Nader at all, he could've debated Nader, why didn't cuz he would've worsened his chances, if you can't beat Bush in a debate i know you can't beat Nader. Also Nader didn't cause Gore to underestimate Bush.
Even DNC Chairman Ed Rendell criticized the Gore campaign's major errors:

1 Not exposing Bush's record in Texas
2 Not taking on Bush's accounts of lying by exposing Bush's Lies
3 Underestimating Bush's ability as a candidate
4 Alienating much of the Democratic base until the last weekend
4a Trying to appeal to a more middle class middle of the road deomographic that's outside of the liberal sphere
5 Refusing to use The President to run on his behalf in key areas
Black, Latino and Feminist voting blocks

There are several more errors but if he would've took advantage of the election by taking advantage of any of these strategies. Don't Hate the Nader hate the Game.