bro you are not going to find anything near berlin, that place and everything around it is flat. your going to have to make it to poland or the czech republic if you want to get to the stuff thats closest to berlin. there may be a couple of 4s maybe 5ish creeks in Hess, other than that your going to have to drive to garmisch (in bavaria) if you want to stay in germany. good thing about germany or europe for that matter are the kayaking clubs and the trips they take.
prague has one of the best places for slalome and play boating, on the west side there is hiko sport, a club and manufacturer. right where they are set up is a swwwweeeeeet canal (side flow trib sort of thing) with sick year round waves. other than that and poland, there aint much near berlin for kayaking on 4s and 5s, let alone 3s. if you want more info let me know, i had lived all over europe for about 8 years, moreso in bavaria. there is also the eiskanal in augsburg.