It'   (1)was a hard working time. --> You wouldn't say this in English. You'd more likely say "it was a hard week/month at work" or "the past few weeks have been really hard work!" :) 

I've (2) had to work hard in the last three weeks. I'm the a head of section. One of my people was on holiday and another got ill at the same time. I worked one for three person(3) --> I'm one person, but I worked for three. I did it! I'm the champion! :)

(1) Don't need an apostrophe (') here. An apostrophe is just for when you leave out words. 

(2) Could also say "I have had" - but it would be more natural to just use an apostrophe ---> "I've had."  

(3) Lots of different ways you could say this in English, but "I worked one for three persons" isn't quite correct :). You could just say "I worked for three people", or "I did the work of three people." 

Hope that helps! Good luck :D