I think everyone learning polish has trouble with these three letters, because we are so used to pronouncing them in English.
Indeed. They shouldn't be difficult because they have close correspondents in English, but they are tricky because of where and how they are used.
Tricky. In much of northern England, the y at the end is a bit like like the Polish y. In southern England, it's closer to Polish i.
But...
The English 'i' sound is more of a glide or dipthong, whereas the Polish sound is a pure monopthong. I was trying to teach someone the differences between a few English words. I got the best pronunciation when I spelt these words thusly:
szijt / shyt
pijs / pys
I only wanted someone to pass me a sheet without them thinking I was swearing or being disgusting. Ah the things we do for a little p!ss and quiet.