'It's pissing me off' is the strongest, but also the most impolite (so know your context). It might offend a few people, but not everyone, in the U.K. It sounds like you're not just annoyed, but getting angry. 'It's getting on my nerves' is the informal way of saying 'It's irritating me' (or, better, 'It's annoying me'). To make this stronger, you can say 'It's REALLY getting on my nerves' (emphasising the 'really' with your voice), or even 'It's really, REALLY getting on my nerves'.

it also depends on your way of speaking and tone. I mean your gesture and posture.

I agree. 3 is the strongest in terms of emotion, when you're really upset, though if you use it with strangers instead of with friends/people you know, but you also run the risk of starting an incident. If you meant strongest in terms of sentence structure, they're all very sound. On the opposite end of the scale (soft instead of strong), you could say "It's really upsetting me." Which would imply it is almost hurting you emotionally.

Ditto what Susan says. "it's getting on my tit's" = old like "pissing me" off but not quite as strong. It refers to mans and womens "nipples" being sensitive and sometimes being prone to itch. like both sexes sometimes experience when clothes they are wearing gently rub and itch and irritate the "nipples".

No 3 is the strongest, then 1 and then 2. However, no 3 is not very polite - you would only use it with friends or people you know well.