They compete for food and the smallest one will stay the smallest one. Its better to keep them separate to make each gecko as strong a possible. Especially for breeding purposes.
I've tried keeping 3 girls in a 55 and sure enough the smallest kept loosing weight. When I separated her she beefed up like a champ. I put her back in with the rest and the middle girl started loosing weight. That's how I came to my conclusion.