As alexander states, the third option is correct. Ex: If I save lots of money, I can buy a house. Vs. If I save lots of money, I will buy a house. The difference is small, but "I can....(do something)" = It's a possibility.meaning I don't Have to spend my money on a house, BUT I CAN if I want to. As opposed to : "I will....(do something)" = it's a certainty, in the future I WILL save money and I WILL buy a house. I Will= similar to "I intend to....(do something). Other examples: Tomorrow I will go to my friends place. = tomorrow I intend to go to my friends place. Tomorrow I can go to my friends place (but I may be busy) This weekend I can come over to your place (but I may not) This sentence pattern is really common. You can even add a "clause or reason" = but or because: you will or can do somthing. I can work tomorrow, BUT I don't want to. I will go to the shopping centre tonight, BECAUSE I need to buy food. Hope this helps.

It's not possible to put two modal verbs in the same statement, so "will can (buy)" is not correct. Your options are "will be able to" or even just "can", if the context tells us it's clearly about the future. If I cash this cheque, I will be able to buy a new car. If I cash this cheque, I can buy a new car. It is possible to use "if I can..." in conditionals, but can/could are really the only modals that can go in the if-clause comfortably. Normally, we use simple tense. "If I can cash..., I can buy..." is possible, but the repetition is distracting.

The Trird one is correct.

the 3rd one is correct .

There are two definite answers. I can't prefer each of them.