Well,all of these three words can be translated to"if",and all of the three are commonly used words in our every-day lives,but they can be used differently,according to the situations and what you wanna express."要是“ is more commonly used in spoken Chinese and may sound a bit weird if you use that in written Chinese especially on formal occasions.On the other hand,假如is more commonly used in written Chinese and formal occasions and it might sound weird if you use 假如 when you make a random conversation with your friend, 如果 can be used both on formal or informal occasions .For example,you can say to your friends either"我要是学习用功肯定能通过上次考试“or 我如果学习用功肯定能通过上次考试（I would have passed the last exam for sure if I studied hard).But it sounds a bit weird if you say我假如学习用功肯定能通过上次考试.And if you write an essay or anything in Chinese with the title :If I were a millionaire,then you can either say 假如我是百万富翁，or 如果我是百万富翁，but it sounds a little strange and very informal if you say 要是我是百万富翁。

要是 and 如果 are very common in chinese communication, they always can be replaced by each other. 假如, in my opinion, although different from the upstairs, is a bit of normal. i usually see that word in article or some crucial moments, for example a well-known sentence in China, 假如上天再给我一次机会，我会对那个女孩说三个字，我爱你。(if the god gives me another chance, i will no doubt say i love you to that girl) as to the grammer, these three words lead to adverbial clauses of condition, remember putting them in the front of a sentence

你看得懂中文吗？个人感觉三个词基本都是一个意思，都是常用语，可以表示1、对未发生的事件的假设，2、以某种事件作为条件而引发另一事件。 比如说： 1、（要是/假如/如果）你看得懂中文，我就不用解释得太费劲了。 2、（要是/假如/如果）你给我100块，我就不告诉老师你干的坏事。

== 我一个中国人觉得没有很大差别。

They are all the same but "如果” is more formal. And in terms of "要是”，it seems like we use it only in speaking.