i need help in constructing my thesis, so far i have two sentences and still more to link in it.

i have decided to use hitler, roosevelt and canada as my case studies.

so far:

Nationalism can be used to unite nations against enemies for the preservation of democratic civilization like Roosevelt. However it can also be exploited to fuel an expansionist voracity like Hitler.

would it be okay to just link them together with a comma between Roosevelt and however.
but then i don't know how to link canada in.
i am just basically say how canada is unlike past nations and how views of nationalism have shifted.
especially in such a culturally diverse country as Canada.

You are trying to write your whole introduction. No wonder you're having problems.

A thesis statement is just that -- a sentence. One sentence. So this is the most important part of what you have written above:
" ... basically say how canada is unlike past nations and how views of nationalism have shifted."

Focus on writing THAT set of ideas into ONE sentence. Then you can add other information in the rest of your intro and to develop all these ideas in your paper.

Here is the very best website I've ever seen that shows you poor and revised thesis statements:
http://leo.stcloudstate.edu/acadwrite/thesistatement.html

To link the ideas together and include Canada in your thesis, you can rephrase and expand your sentences as follows:

"Nationalism can be used to unite nations against enemies for the preservation of democratic civilization, as exemplified by leaders such as Roosevelt. however, it can also be exploited to fuel an expansionist voracity, as observed in the case of Hitler. In examining these contrasting examples, it is crucial to consider how the perception of nationalism has evolved, particularly in culturally diverse nations like Canada. Canada stands apart from its predecessors, showcasing a unique perspective on nationalism and highlighting the changing dynamics of this concept in a multicultural society."

By using transition words such as "however" and "in examining these contrasting examples," you can effectively link the ideas together. The mention of Canada and its unique perspective on nationalism adds another dimension to the analysis and allows for further exploration in your thesis.