My son is almost 3 and he is so impatient. He ask me to do something and when I tell him to let me finish what I'm doing he throws a fit and says for me to do it now. I don't give it but he still throws a fit every time and starts throwing things.

The longer you ignore him when he throws a fit he will soon discover that is not the way to get what he wants.
When he settles down, I would then go to him and say "now that you have stopped misbehaving, what is it that you want"

Hang in there and do not give in to tantrums

Reward systems also work well. Come up with a small treat, a sticker or a jelly bean-something like that, that he gets whenever he does wait patiently without throwing a fit.

First of all, if he throws a fit he gets NOTHING. Not even later.

Second, make sure you spend time with him before you need to finish what you are doing. To a degree, he needs your attention and he is trying to get it. Give him a chance to be a good kid and he will. I know you have other things to do, but hang with him first. So when you want to do something that will take 30 minutes or so, spend 10 minutes with him, stacking blocks or reading or talking. THEN take your 30 minutes. (And tell him you are going to need to focus on something else for 30 minutes and help him get settled doing something on his own but near you, like continuing with the blocks. Be really clear about your expectations and Keep Your Word.)

Third, give really clear signals. If he asks for something either say yes or no, not maybe or "later." If you say yes, then say when it will happen. For example, say, "I need to finish this. It will take me 10 more minutes. I know that feels like a long time. But when I'm done, we'll go outside and throw the ball." Or, "We will go the pet store to look at kittens at 3 pm this afternoon. We are not going this morning." If he cries, that's fine. It's your job to be the bad guy. Just accept that. ;)

I read somewhere to give kids "red lights" or "green lights" but no yellow lights. This really helped me with my kids when they were small. I recommend thinking this way. Whining often results from mixed signals. Yellow means "yes" if you whine or pitch a fit, "no" if you are good. You don't want that!

Good luck with your son. :)

PS. He's little. So don't expect him to wait, like, 2 hours. I would guess 50 minutes is probably the maximum for a 3 year old to manage himself.

3 years old is still pretty young. An hour is a long time for them to wait.
As parents it is sometime hard to prioritize what needs to be done and what we want to do.
Make sure his tantrums for attention is not because he does not get enough to begin with rather than just wanting to have his way.