I have 3, 5 volt 200 amp switching supply's they all work, however when I try to wire them in series to make them into a 15 volt power supply 1 drops out. 2 will regulate with each other, any 2 but when I add in the 3rd one one will drop out, Any suggestions on how to over come this problem ?

 

Sounds like a disaster waiting to happen.

Read this web page and also others when you search for "connecting power supplies in series"

 

You need to get a 15Volt psu on its own, its not good practice to use several psu's together, the reason is that they are not isolated from ground so they short out.