I was accepted for a PhD position three months ago for a Marie Curie grant. My problem is that, doing a PhD was not on my career plans but, because there was no other good opportunity and my degree is a research based degree, I felt forced to accept this. A Marie Curie grant also pays good money, so that was important. The thing is, I'm three months into it now, I am not enjoying it at all and I feel stuck in this decision of whether or not I should continue it and see if I start enjoying it. Can someone advise me on what strategy should I try to see if I can improve my motivation?

A lot of people I knew weren't enjoying their PhD at 3 months - myself included, actually. Then it got better. It's worth hanging in there if it is what you want. On the other hand, if you think you will want to quit it every time things get tough, then you may as well quit now since there'll be plenty more of that ahead!
Bottom line - do you want to do your PhD? I think you will start to enjoy it if you persevere. : ) At this stage, maybe talk to someone you find inspiring or motivating - an old advisor or lecturer for example. That could help. Also, with the tasks at hand (e.g., lit report) set yourself small achievable goals/sections. Good luck!!!