The first sentence is correct, but the rest two are incorrect expressions.
我是要去法国的 means I DID want to go to France. Good work. Pay attention that this phrase is used to emphasize things happened in the past, so I use 'did' here.
However the rest two are incorrect in grammar. When emphasizing a finished action using 是, it should be put ahead of verbs. So here 是 should be put in front of 要去, and any other places are unacceptable. Some other examples: 我是骑自行车来的 - I came BY bike；我是准备给他打电话的，可是手机没电了。- I did intend to call him, but my phone was out of power.
What interesting is that if you put a "话" behind the second sentence（as 我要是去法国的话）, it will become idiomatic correct, but the meaning will completely change. 我要是去法国的话 means 'if I go to France, (then....)". "要是" is a word indicates assumption, it is a colloquial way to say "如果(if)", and "的话" is a modal particle indicates subjunctive mood, it is attached behind an adverbial clause of condition which begins with "如果" or "要是" or other words that means "if". And, since it is a modal particle, it can be omitted. A standard example should look like this: "如果...的话，那么..." - "If..., then...". Nevertheless, it can be flexible in concrete forms when used in daily life.
And for the third sentence, to express 'I'm going to France rather than anywhere else", you can say like this: "我要去的地方是法国。" This is actually translated to 'The place where I want to go is France".