Question regarding post-doc salary and moving expense (US)
In my experience, the particulars of a postdoc position vary more widely than academic jobs at any other level -- so much so that it may not be helpful to speak in generalities.

I have direct experience with (some) postdoc positions in mathematics in the United States and Canada.  For these postdoc positions, the salary was the same for all years (usually 2-3) of the position.  Moreover this salary was a standard one given to all postdocs of the same "kind" (roughly, the same funding source) and was not negotiated in the cases known to me.  However, if I remember correctly, the last time I brought this up on this site someone else told me of a North American math postdoc position that did include annual raises.

Again in my (relatively limited) experience, someone who does a second postdoc is not "senior" in any positive way to someone who is on their first postdoc -- if anything, there may be a mild perception that someone who is doing their second postdoc was not successful in landing a tenure track job.  But these things may be different in your field.


  how can I 'politely' ask them to raise the salary according to 3rd year post-doc level?


The way to ask politely is...to ask, politely.  Namely, don't ask them to do it, but inquire into the possibility of doing it.  There is really no harm in asking about a salary bump, moving expenses or whatever -- if it's not on the table, they'll tell you.  If it is, you can explore it.  As long as you don't make hard demands or counteroffers, they should be happy to talk to you about it.