DC DC Converters in Parallel for double current
Shorting the outputs will be problematic. They are voltage regulators and the one (of the three) that wants to regulate at slightly higher voltage than the other two will end up supplying most of the current to the robot. It's best to choose a dc to dc convertor that has better spec.

Alternatively maybe you can, with relative ease, redesign some of the robotics so that they can operate directly from the LiPo battery - There is a decent chance that this might be easier - using a DC-DC convertor running from about 15V and supplying 15V makes the convertor design more complex and no-matter how efficient it's spec says it will waste power. If you can look into the robotics and decide which parts can run from an unregulated voltage this might be better.