Trying to make a Nested Template with a 3 column grid

You do not under any circumstances want to make a NESTED Template.  Templates within Templates are counterproductive & nothing but headaches.  I can't think of any scenario where an ordinary template won't do exactly what you need.

Ideally a Template contains everything you need for your entire web site with optional regions for things that may appear on some pages but not on others.   Your sitewide CSS contains all the styles needed for the various layout situations.   It takes careful planning to create a good site template.   But doing it right will save you time in the long run.

A 3-column grid is no great mystery.  Can we see your code so far?  A URL is preferred.  A distant 2nd option is to copy & paste everything into a forum reply.   Don't use email, it won't come through.

Nancy