3 MCP355: 3 x 18W (max power consumption) = 54W.

The maximum heat dump can never exceed the maximum power consumption. A lot of the energy gets converted into kinetic energy, the rest gets dumped into the water as heat. I'm not sure what the actual heatdump of a MCP355 is, but I don't think it's nowhere near 18W. So even if it would dump 18W, it still isn't that much (CPU and GPU usually dump double that when under load).