If you can't focus for five reps, I'd work on that underlying issue first. It's, what, an extra 15 seconds more than a set of 3? C'mon. I would suggest buckling down, doing things you "hate", and following the program as it was written because it works for what you're looking to achieve.
For sure you can build strength training with 5x3 instead of 3x5, but I think your reasoning for wanting to change isn't a valid excuse.
You weighed 63 kg in the beginning of October, too. I'm going to go out on a limb and say you very rarely, if ever, eat 5300 calories in a single day.
It's good that you're trying to get your training and diet together, but what makes you think you need such crazy-high calories for a kid your size?