Is 3+2 the most competitive mode?

Hello,

I had an ELO of 1500 playing blitz 3'. 

Then I started playing blitz 3+2 and my ELO dropped to 1300. 

Is 3+2 probably the most competitive mode on blitz 
(I have 1500-1600 in bullet and rapid)?

There is no way to really tell that. 

It looks like your rating has fallen from 1491 on July 21st.  Then you played about 150 games of 3 0 and lost 100 points.  Then you switched to 3 2 and lost another 100 points.

You get different groups of people playing at the different time controls.

Another big factor was the fact that you made the Legend advance on July 24 and then cut way back in playing.  Undoubtedly the grinding you were doing to make the last advance affected your rating.