The LP and the constitution party have some things in common, but the greens are too socialist to have much in common with the other two. Although ballot access and opposition to the war are positions shared by all three.
I've taken a slightly new direction; persuade Ron Paul to run for president as a Republican.
He's too old to run an effective campaign.
Look at your options; McCain, Bloomberg, Guliani, ... Hillary, Biden, Kerry etc. Who do you see among the Republicans or the Democrats that you would want?
Admittedly his chance of getting nominated is small but its better than the chance of a Libertarian getting elected.
Even if he kicked the bucket while in office he would still be a better president than the others.
BTW even if you could get a coalition party of the Green , LP and constitution party you still could not get more than 3 or 4 % of the vote.