For 3 people, no contest, take a taxi (€35-50 depending on traffic, time of day, # of suitcases).

Shuttle services run around €20 per person, making them unattractive for a party of 3.

You could take the Air France bus to Etoile (€10 one way), and take the subway or a short taxi ride to La Défense, but the savings will be minimal.

Taking the RER train and transferring to the metro will only cost €7.70 per person, but with significant aggravation.