Can I ask to be the first co-author? I am working in a lab as a research assistant for the last 3 years. 

During this time, I published three papers (1 first author) and three of them were published in good journals. In one of my published work (I am a co-author), I had to repeat almost most of the works (writing, figure, table). The project was done by another person and rejected several times. Then he went for a Ph.D. and he did not bother and did not continue with that project. As we have the same supervisor, so the supervisor asked me to work on that paper. I had to correct a lot, had to do experiments again. Then the paper got published. Even all the major corrections (that reviewers asked was done by me) as the main author had no idea at all. That time I was thinking I should be the first coauthor as I worked really hard on that project. That project was kind of dead and I started to work on that after two years and it got published. 

Now I have two more projects like this (both were done in 2015 and none of the authors care about them anymore). Again I have to give the same efforts as a new project. So is it worth to ask my supervisor to make me the first co-author? Is it too much to ask for? 
I am happy that I get many projects to work on, but personally I think I should be the first co-author as I have to work hard to get them in shape.

PS: My supervisor is very pleased with my work till now.
Yes, of course you can ask. But in some fields there are other considerations that are considered important and first authorship is assigned for reasons not obvious to people from other fields. Sometimes those reasons are just political, but people go along with them to keep peace. It might also be necessary to keep the peace with a supervisor who has authority over you and can affect your career. 

So ask, even if you might not get what you think is fair. Think long term and preserve relationships. Having a publication, even if not as first author, is still valuable. 

And there may be future work that you can extract from the project that won't bump up against the same considerations, making your own first authorship the obvious choice.