You cannot set iron sights in these, you just have to aim higher to shoot further, it's pretty intuitive and you just need to practice. Keep in mind, they are both very primitive wepaons, especially panzerfausts which effective range is extremely limited anyway. For example, panzerfaust 30 has a range of 30m, Panzerfaust 60's range is 60m and it's 100 meters for Panzerfaust 100, hence the name. It's a bit easier with PIAT, it has the iron sights set for different ranges, like mentioned above, you just aim through one of them to hit your target.