can a weak cipher in a cascade weaken the result? So i am a real beginner .
And I use AES(TWOFISH(Serpent))) for my veracrypt file . 
So I asked myselfe a Question : 

When a Cascade is made the Key is split in parts for each algorythm . 
So if We have AES(Twofish)) with the key : Test-password123 The "Test-pass" Part would be used for AES and the "word123" part gor twofish right ?
So if you would use something like rc4(AES)) cascade , The rc4 would weaken the AES cipher right ?
I'm tailoring my question specifically to VeraCrypt where needed.


  So if We have AES(Twofish)) with the key : Test-password123 The
  "Test-pass" Part would be used for AES and the "word123" part gor
  twofish right ?


No.
VeraCrypt (and any other somewhat decent encryption program for that matter) uses a so-called "password-based key derivation function" (more precisly PKCS#5 PBKDF2), which takes the full password and iterates a moderately fast function (a so-called hash function) very often. This is done so an attacker can't easily verify a guess for your password. Now the result (a sequence of bytes) is taken and split into different sub-keys (2-6 keys in the case of VeraCrypt)1.


  So if you would use something like rc4(AES)) cascade , The rc4 would
  weaken the AES cipher right ?


It depends, but in general no.
AES, Twofish and Serpent are so-called block ciphers, that means they take a block of (16) bytes and transform them into a different block of (16) bytes using the key, such that the transformation is reversible given the key and not if not given the key.
These block ciphers need to be run in a so-called mode of operation (XTS in this case). There is no known attack that performs (significantly 2) better than brute-force key search for this cascade, so you are required to perform at least $2^{256}$ operations which is infeasible.
In your concrete example, it depends on the mode of operation for AES. If this mode is a so-called stream cipher then there's a result that states that the encryption is at least as strong as the stronger algorithm.



1: Actually it runs this chain mulitple times to generate a set of key-bytes each time and then splits.
2: Actually double-encryption is only slightly better than single encryption due to the meat-in-the-middle attack.