I wonder if the 3 is a mistype!
Not a mistype. In English you can also write 4you. 3
maj means trzy
maj, that is trzymaj się, which can be translated as take care.
So the whole translation of Jestes moim najlepszym kumlpem I ciesze sie ze cie mam..!!3maj sie caluski
You are my best friend and I am so glad that you are mine..!! Take care. Kisses.