I've been seeing a lot of threads and comments recently about people worried about their 2017 tax returns. I don't think there's anything to worry about. If you are applying now, and have not done your 2017 taxes (because of course you haven't - nobody has), just tick the "no" box. You're allowed to say no to things on the application that you don't have, because that's the truth. You still qualify with 3 previous tax returns. And by the time CIC gets around to actually looking at your application and cross-checking with CRA you'll probably have filed anyway. Which doesn't even matter because you'll still qualify regardless!
There's too much stressing going on! This is the "easy" part of the Canadian journey guys! We all struggled through the uncertainty of trying to get PR that I think the stress of dealing with CIC again has been re-triggered in a lot of people. Whether this process takes 5 months or 5 years, we're not in danger of losing our work permits or being deported because time ran out. Faster would be nice of course, but the stakes are lower. Just answer all the questions honestly. The requirements are all clearly spelled out, and if you meet them you will succeed in time.