I have serious gaps in my knowledge for maths, physics, chemistry and I need to study each for at least one year
can I apply to universities with 3 A-levels that have been acquired in three consecutive years? let's say A-level maths in 2015, A-level physics in 2016, A-level chemistry in 2017
or the universities will not take me seriously?
You can apply to University having done the 3 A-Levels in 3 years. However, taking so long over doing them will hold you back when it comes to going to the top-tier universities, since doing them in 2 years show that you can cope with the rigours of focused academic work.
My advice is that you should do them in 2 years. Your university prospects will greatly increased, and the workload is manageable (assuming that you don't have to wok full-time).